I've always figured one reason is the open world, and that's the reason EA decided to introduce the semi-open world in TS4 so it would perform better. All the Sims games are buggy, but TS3 is hands down the most. I've played TS3 on several machines of various specs and haven't had too many issues other than mild lag. I probably haven't played it enough to experience anything major, though. Out of 2, 3, and 4, I've played 3 the least.
With TS2, TS3, and TS4 all installed on my current computer, I prefer TS4 because of its performance. It runs so much smoother, but the game itself is less complex (obviously a major complaint of TS4 players and the reason a lot choose to play TS3 instead). I think they tried adding too many features to TS3, which set the bar high for simmers. Now they're focusing on performance to allow more people to play on lower end machines.
