Because it's really really good art, great characters, great story, etc.?

Because it's in the nowadays very rare category of things that have a real plot but dosn't build it on sex/violence/grimdark?

Because it has antidepressant properties and teaches lessons that are often valuable to a rationalist?

Because it's very different from everything else you've seen, and keeps subverting your expectations?

Because you find pleasure in subverting meatspace expectations, and with this you can comfortably do it in a way that's still acceptable from other contexts?

It's adorable! It's happy!

And however it might rankle the physicalist majority, I think the most parsimonious explanation is that Friendship Is Magic.