It's not easy as in, MK doesn't play itself.
I main Jiggs, learning her is way, WAY more difficult and even if you master her (which I haven't done yet), you are at a huge disadvantage against ton of characters. And it's not a bottom-tier-only thing - Snake has a very difficult time against DDD and Marth, Diddy gets owned even by Luigi and Peach, Falco is destroyed by Pikachu, etc.
Learning MK is awesome because it DOES take less effort, and he does great against pretty much everyone. As such, there are tons of MKs and again - the game revolving around one single character is wrong.