I became a Rumble main back when he was extremely, ridiculously, absurdly OP in season 2 (imagine if his Q did significantly more damage and his ult did a huge chunk of instant damage), and even then there weren't many Rumbles around. There's a lot of things that make him unpopular:
- Unique Heat mechanic that, if you mess up, can silence yourself at a crucial moment. Until you master it you can often feel like you're playing him badly by missing overheat opportunities or not using the right enhanced abilities.
- Ultimate has unique targeting mechanic and you will be blamed if you use it badly (like Ashe arrow, but worse). Sometimes you'll be blamed when you use it well too.
- Neither an engager, or a 1v1 champion (especially later in the game). Main tactical niche is finding/baiting and winning teamfights.
- Melee, immobile and has weak early lane phase. Therefore vulnerable to ganks and being caught out.
- Until you master him, your CSing will be awful.
- Only played top lane, so doesn't expand your mid/jungle champ pools like many other top lane candidates do.
I don't think Rumble is particularly hard to learn, but there's a lot of odd things about his kit that make him require more investment than others, and the rewards for doing so aren't as immediately obvious as with the other premier top laners right now (fighters like Irelia, Riven, Ryze, or the tank-god Maokai).