Everybody will answer "greed, racism, idiocy, and bigotry" or some such rubbish, because morally and overall psychologically, that's the most comfortable answer.
The real thing is somewhat complex, and most people won't buy it.
Of course, part of it is those things, but there's way more going on here, some of it is cultural dynamics, some of it conscious intent. Those specifics are the symptoms, not the disease (though they may be diseases in their own rite).
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structural weaknesses in the US government, which was barely meant to handle the complexity millions of people, much less tens or hundreds of millions of people. I.e., bandwidth issues. As more people push their views and goals into the system, all of that needs to get governed or implemented somehow. But there is no cohesive operating principle that guides US (and even other western) culture. There is no razor - not even material necessity (staying in-budget, or managing debt effectively) is accepted. There is no means to trim implementation that all parties will be happy with, so things don't get trimmed. They get crammed in, the laws (in the sense of legal structure, not crime) are consequentially self-conflicting, improbable, or impossible to fulfill. This leads to an intrinsically unstable environment, ripe for (and rife with, by all parties) abuse. What you are seeing is, in part, the breakdown of the rule of law. This breakdown can be allayed, to some degree, with authoritarian means, but that only goes so far, even if that authority has a willingness and capability to work with the people as a whole - which none of the active authorities do, anyways, except maybe Bernie, and he's been written off by the authorities because he can't work with them well, and they also have valid concerns that must be addressed. But, in any case, whether centralized or not, this breakdown is to be expected, because the rule of law, unless supplemented with common principle, becomes.. well.. legalistic, and rife with abuse.
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governance that doesn't match underlying principles: we have no conscious least common denominator. People often point to distinct nations and say things like "see? they are doing X right!", but that nation has a cohesive culture, and isn't dealing with anywhere near the level of cultural complexity that any melting-pot nations are dealing with. What is enforceable must be agreed upon by common culture - or you must sacrifice the reality (though not necessarily the pretense) of diversity, and enforce your way. But that has obvious flaws. Instead, it is better, in my opinion, to enforce sovereignty, which is intrinsically what all the different cultures want, anyways, except that they also want to take control of everyone - which they don't get to do in a system with sovereignty as a basis, except by people ascribing to that culture. What you are seeing, is in part, a breakdown of unity due to a lack of agreement about what can be universally enforced. I.e., the system implemented does not address underlying cultural commonalities.
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the need to incorporate raw power and personal responsibility into the governing body. Bending the rules, breaking the rules with impunity, changing the rules, explicit and implicit coercion are all possible, and as such, the existing system or ruling party must be able to address these things, and incorporate them where needed, for the larger good of upholding the spirit of the law. This relates to the breakdown of the rule of law, but is more primal: you know raw power must be met with raw power. That power can be of a different form, but it must be effective.
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unconscious cognition of complex truths: or, in some senses, the "vote of no confidence". People understand, or are at least impacted, by the above issues. They have instinctive reactions against external control, and for good reason, as individual sovereignty is the source of a solid collective. But in any case, many people are aware there is a problem, don't see a solution, and are see no option but to let things burn. This may not even be a conscious choice, but simply an overall feeling - and thus, more powerful and deeply-rooted.
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genuine mockery and rejection of opposing views. Nobody gets each other, unconsciously, and everyone else treats others outside their worldview like shit, and pretends that doesn't matter. A lot of the left separated from the "Christian" right due to this - only to turn around and do the same thing to the center and right, feeling just as justified in doing so. But it creates real alienation and aggravates the already deep wounds and rifts that exist. One's personal actions, thoughts, and feelings may not seem to matter, but they resound loudly in the whole - and making personal change does, too. For those who are genuinely growing and facing their hearts and minds - my respect.
All of these contribute to Trump's rising and staying power. Of course, he's just riding a wave of unconscious thought, and if it weren't him, it'd be someone else. But people like to fixate on a face.
The actual thing we're trying to do (integrate diversity into a cohesive whole) requires genuine acceptance and support of differing world views (including non-scientific or non-Christian ones - why do I have to say this?). That means that your group, your ideology, must make room for the people who are "wrong", and wish to live their lives wrongly in abhorrent wrongness - though they never gain the right to enforce participation in their culture, above and beyond what is a natural requisite by birth, upbringing, or other dependency.
That is, each person and organization has a sovereign right to rule their own life and the lives of their dependents as they see fit, but does not have the right to force others to use their system, nor to prevent others from abandoning their system and starting their own or joining another. This integrates the very opposite of federation (well, not in the Lemmy sense, which is actually confederation, but that's a no-no-word because some people thought that confederation did give them the right to force others through slavery - but it doesn't).
But Sovereignty Culture isn't simply confederacy, like Lemmy is, but it heads towards the same things. That which can be federal is only that which we fundamentally agree on. The federal must not be used as a means of furthering ideologies, but as a means of resolving disputes between differing ideologies. It can have as much power as the people grant it, and no more - else it loses the people. By making sovereignty a keystone of culture and governance, we intrinsically grant and naturally enforce rights of others, but without placing a burden on others (except the burden of self governance, which you already have, and can't avoid).
Time.