PSRs where decided based on the most common standards used by some of the biggest PHP projects around. It wasn't a single person setting a standard. PSRs are completely optional.
However, you miss the point. The point of this isn't really to define all the options. Rather, it's to spell out best practices (and PSR-0 is really a best practice, and all about interoperability) that the community generally agrees with. Better to have a standard coding style than to have none at all, and if you have to choose one, PSR-1 and 2 are an excellent choice (considering how it was devised).
Im sorry but accepting underscores as a valid alternative to namespaces in 2012, is not a "best practice".
The whole concept of PSR-0 is ridiculous anyway, because PHP supports registering multiple class loaders. If a project wants to use a naming convention that won't work with standard spl_autoload(), they can register their own autoloader.
except if you've ever had to work with lots of different autoloaders at once you'll know that you regularly get conflicts, and getting the correct autoloader order is often a case of trial and error. Using sane conventions across libraries avoids all of this mess.
The reason for underscores as namespaces is simply that some older libraries want to maintain compatibility with 5.2X for whatever reasons. Of course it's encouraged that newer libraries are written with proper namespaces and almost all of them are.
However, you miss the point. The point of this isn't really to define all the options. Rather, it's to spell out best practices (and PSR-0 is really a best practice, and all about interoperability) that the community generally agrees with. Better to have a standard coding style than to have none at all, and if you have to choose one, PSR-1 and 2 are an excellent choice (considering how it was devised).