Maybe not an eli5, but lots of reasons.

There's no stable, consistently updating client that everyone agrees on, the real 'emule' client hasn't been updated in over a decade. Once you get past that hurdle, the setup is also a lot more cumbersome than other file sharing options. The network also has kind of a bad reputation because there's not a great way to see if you can trust a file until you're finished downloading it and people definitely do take advantage of that.