Because OpenCoin is creating a large, complex piece of software which will be released as open source. So in exchange for the large "premine" they are creating significant value.
The additional value that SolidCoins could offer was very little in comparison to that.
Satoshi somehow managed to create a large, complex piece of software which he released as open source immediately without a pre-mine. How did he do it? It sounds like he needs to take his investing advice from OpenCoin from now on. Their pre-mine is 100%. Will they be putting in 100% of the effort into making Ripple a successfully currency?
So what are they? Why users are not allowed to create their own IOUs?
They are a 100% pre-mined currency that you are forced to use and hold if you want to participate in the Ripple network. The reason is to make OpenCoin rich. That's all.