Many things, but this isn't really one of them: The reason Sims is open season for modders is not specifically because of anything EAxis did, but because of the way the game is designed: The Sims is a modularized game where things are object-oriented and gameplay is derived from the interaction of scripts. Splotch is a linear game where the gameplay itself is essentially the same no matter what you choose: The modular components exist only in the collection of potential creature and vehicle parts, and some global configurable parameters that can be tweaked. Much like how core gameplay of the Sims is hardcoded, the same applies to Splotch.
Correct me if I"m wrong, but Sims 3 is not being coded that way. I don't get the impression that it will be open season for modders at all, because they provide "tools" within the game to do stuff. The tools are nice, but they are very limiting compared to what modders could do for Sims 1 and 2. This is another reason among many of why I'm not looking forward to Sims 3.