Well, as far as the ult goes, as that is always what's brought up about Urgot, it's better than people give it credit for. But it isn't useful in its original design except for fringe cases, that being catching people out and putting them into your team. Vision was a much lesser concern back when Urgot was designed so catching people out was a lot easier and didn't always leave you in a 1v4. Now it's more useful in 1v1 situations against carries. The damage prevention it provides is immense along with your passive. And more often then not it allows landing an E for free.
I guess the main reason people don't play him is that he falls off hard late game.. At least, the way most people play him on free week he does. To transition, you have to go from being a carry early-mid to being a utility off-tank late game. This of course leads to a conflicting play style. Late game, all you need for damage is Muramana and an armor pen item, you can pretty much take any of em, they all do good work. After that, just build Frozen Heart, Banshee's Veil is nice, Randuins for those 1v4 ult dives to escape.. Late Game, you should find yourself dealing okay damage, taking a LOT of damage for your team, setting up a lot of kills for your team and pushing towers pretty hard.
Two other reasons:
First, His kit is SUPER risky. Don't land E? Good luck doing any damage against anybody with blinks. Use your peanut shield too early? There goes your perma slow. Use your shield too late? Take a guess. Ult at the wrong time? "An ally has been slain."
Second, he's Urgot. He's ugly, he was gutted harder than Olaf, he sounds worse than a dying crab (oh, wait), and he's a giant green spider crab with man boobs.
And I love him for all of his flaws.
Urgot is love. Urgot is life. 