What do you think is responsible for lemmy’s growth over other alternatives like KBin and Tildes?

Tildes is just too small. The obvious explanation for growth is all of the Fledditors (Rexit? I like Lemmygrants, but that really only covers people who came to Lemmy) looking for an alternative. People wanted a drop-in replacement for what they already had. Tildes didn't even have enough of a seed in their biggest subs, let alone their (very few) niche groups. Same for Raddle, Squabbles, etc. The only subs that made a significant migration to those are the ones that packed up, locked the doors, and left a forwarding address to anyone left - Similar to what r/piracy did, except that went to Lemmy (complete with instructions to ignore the federation questions)

As for Kbin, I think the bigger factor is coverage. As soon as anyone started mentioning people leaving for greener pastures, Lemmy was always the first thing mentioned. Kbin was always a second-place alternative, along with a few others. Since Kbin has the same confusion about federation as Lemmy, it didn't pick up a lot of people that bailed on the first choice.

Not that it matters much anymore, since Kbin is well-federated with Lemmy