Speaking as someone who totally doesn't understand federation (but totally does get servers being overloaded) - I can completely see why they all joined what appears to be the primary instance. I did. I really struggled to work out which server to join and had to wade through a few that had their own special rules (eg "no creating communities here" - idr which one that was tho).

I ended up joining lemm.ee simply because it seems like a nice generic server set up to do general stuff with that wasn't lemmy.ml. Is that a good choice? idk.

I had a similar problem grasping mastodon (actually the reason I didn't really use it in the end).

Lemmy servers need to work more like Counterstrike or TF2 or WoW servers (edit: or IRC servers - that's probably a better comparison tbh), where you might want to join a specific server with its own personality, but most people probably don't care and are more interested in whether it performs well and is likely to be around a while. I also think some simple things like making the server less prominent in the UI and not making local communities the default view would help loads with people not feeling like they're less because they're not on the primary instance.

Edit: LMAO except I didn't. I posted using the account I'd made on lemmy.ml but decided not to use. Lemmying is hard, yo.