For me feddit.uk stuck out as when I was joining Lemmy, (my initial choice was lemmy.world), it seemed like it would be a safe bet as it was clearly set up by UK based people that had also come from Reddit like myself, so it had a sense of familiarity. I'm guessing that's why the other feddit.<tld> also became somewhat popular instances.
Probably because someone started and then all the others saw it and went with it? I'd say it's the same dynamics as if you're the first one parking your car in a (not so obvious) no-parking zone, and you come back two hours later and there is a whole line of parked cars behind your's. It's kind of inevitable, especially after the second person did it...
And I mean it's a bunch of instances named like this. But not an outrageous number... We have quite some diverse names for instances. And lemmy.something is the other super popular choice.