To answer the question in your topic line: Yi's rework gave the community exactly what they asked. Players were complaining about Yi's AP build, whether they were counter-playing him or not. The people who didn't counter-play him were butthurt about getting gibbed all the time, and the people who did counter-play him noticed it was his more viable build, which seems ironic for a melee AD assassin/carry to be more viable built AP.
A lot of players, mostly on the side of losing to AP Yi, kept telling Riot to make Alpha Strike scale on AD. Everyone else was telling them how powerful that idea would be, but the complaints probably got to someone at Riot. This is the point where I'm guessing, but I can just picture two Rioters like this.
"Ugh, this is probably the thousandth time I've seen a thread about AP Yi."
"Seriously? People are still complaining?"
"Not just that, they're saying they'd rather have Alpha Strike AD-scaled."
"That's an insane idea. I mean, can you imagine what that would look like?"
[pause]
[both] "Let's do it!"
No, really though, making Alpha Strike AD-scaled and physical damage was actually most likely to make counter-building against Yi less of a hassle. A carry's offense is designed to eventually outscale defensive building, but you can still counter-build effectively early game.
Seriously, what is the main reason people pick Yi? Cause I can't see it.
Because Yi's a simple pick that's viable. Simple+Viable=Popular (see Caitlyn)