Choice? Some vendors lock you in allowing you to use only their app store.
In a word: fragmentation. Just look at what Amazon have done with the Fire. Separate branch of Android with a separate app store. Same thing happening to webOS would be bad news.
The problem is not having 2 or more app stores, the problem is having 2 or more app stores that are mutually exclusive.
Is there an example of two or more app stores that are not?