A few difficult reasons.

1. People with chronic illnesses frequently don't have the energy to make noise and bring attention to their cause.
2. People with significant disability usually aren't hiding in the halls of power, blending in. It's significantly easier to hide a non-straight sexuality for long enough that you can be the decision maker who makes reform happen.
3. Disability is so varied, one person with one disability can't know what it is like to have a different one. This stymies cross-disability advocacy. This gets even harder for family of people with disabilities, who only conceptually maybe understand their loved one's conditions, let alone other people's
4. Accessibility accommodations can be complex and can be expensive, LGBT+ inclusion is extremely easy and low effort by comparison.
5. Understanding how disability affects daily life pervasively is harder than understanding "those two people are in love". Most people internally assume everyone else has roughly the same abilities and needs as themselves.
6. People with congenital conditions are frequently conditioned into not asking for better treatment. They get used to being second class citizens because it's all they've ever known.
7. "Coming out" with a disability casts doubt on your ability to "perform" in the workplace. It's very risky, the stigma of disability is huge and impacts how people evaluate you
8. Most people with disabilities don't consider themselves disabled. For example, glasses are one of the world's most common disability aids, but you'll never hear them referred to as such; and rarely will their users consider their poor eyesight a disability. Mobility disabilities are often written off as "I'm just getting old" or "I'm just a little unfit lately" or "it's just an old injury playing up" instead of "I have a medical condition which limits my daily life".

There's more, I'm sure I'm forgetting a bunch.

Edited to add a huge one:

1. People who are born with disability or who acquire them at a young age are frequently unable to access the type of formal education that allows them to become professionals. This makes it even harder for them to be visible in workplaces, because they're considered "unskilled". It also means you won't see people with disabilities regularly in offices. Out of sight, out of mind for everyone else.

And another:

1. Money is medical privilege, and the people with the most money make the types of decisions that affect larger quantities of people. For people with disabilities who are born to wealth or who are able to acquire it, their medical conditions can be better treated, managed and therefore hidden. This can result in those people and the people who surround them to not feel like the illness is "a big deal" and hence it's not a priority issue. It's when people with significant money are negatively affected that you begin to hear about the barriers people with no money experience all day long.