I think, in the pony fanfiction circles. It's because of these reasons:
.Writing crossovers is easier, seeing as you can just add in a random thing and ignore making an original story
.Crossovers are generally known to be more popular on sites like FiMFiction, even if they're worse than other fics
.They like both shows and want to cross them over for a bit of novelty
And to be honest, a LOT of pony crossover fanfics are shit as shit can be, the only really decent one I can think off is Fallout: Equestria and that's only really a crossover in name and a few scenarios.
As for drawings, I guess it's because it's much easier to make an original pony that looks like a character because ponies all use the same basic template, all you have to do is add details to make it different. I've done a few crossover drawings, my best one being the Walking Dead. I didn't do it because I wanted to "ruin" anything, I did it because I love both shows and I want to cross em' over because I want to.
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