1

Anyone who has regularly followed sites like Danbooru and Gelbooru will have noticed a radical shift in subject matter over the last couple of years. Used to be about half of it was Touhou Project, but these days that's pretty rare and the most common subject is Kancolle.

I think the reason is pretty obvious: the Touhou characters all look like they're 5 years old, but the Kancolle characters are grown up (and out!). So the tldr answer is "Boobs".

2 To be fair, that's the correct answer to many things.

3 There's a range in both games. Touhou has a couple of lolis and a few adult women, with the rest being teenagers, being drawn as older teenagers in ecchi art. Kancolle has destroyers and submarines generally as middle-schoolers, cruisers generally as high-schoolers, and the battleships and aircraft carriers generally as older teenage and adult mikos. (One of the carriers is younger and has breast envy.)