What everybody else said here about the system specs. The main problem is the Cell proc. It is 6-cores, vastly different from the current systems. It is hard to say whether each of the cores is slower than the 360's cores, although I suspect they are - otherwise everybody would just stick to single-threaded games.
Anyway, besides the 360's architecture being very advanced considering that it came out a couple of years before the PS3, there is also a problem of the dev support. Microsoft has spent the last 15 years working on development environments and APIs (DirectX). These APIs are well known in the game development world. The system architecture is more familiar. The dev environment is just plain better - software is Microsoft's bread and butter, after all.
Also, designing stuff for multi-threaded systems is a pain in the ass. Unless you know exactly what you are doing, it is really easy to get into a lot of trouble. Couple this with a brand new and vastly different system (360's are a lot closer to PCs), limited developer support, and lack of an advanced and well integrated developer environment, and you have your answer.
Furthermore, as a result of all of these factors, games take a lot longer to design - and also more expensive. If for example a game is being developed for the 360 and PS3, I suspect that the 360 version will be finished sooner. Which means that the PS3 port will not be as polished as it could be.