One - Ivy got greedy. Had it make it appear as though he was losing some of the games, he probably would've gotten away with it. If you lose, then win, then lose and are up at the end of the night, it's more plausible you weren't cheating. The way all of the articles I've read about it, he basically ran the table for several hours and then left.
Two - you go into any casino and use card counting techniques (like the ones the MIT team used) and within ten minutes, the pit boss will cut you off because its obvious what you're doing. This is the very same technique Ivy was using. Lowering his bets when the cards were low, then increasing his bets when the cards were favorable. I'm actually pretty shocked the casino let him play for so long when it was so obvious what was happening.