[djur]

There's a lot of reasons, but the most significant one is that Ruby successfully and recently made a compatibility break between versions (1.8 and 1.9), which made the ability to run multiple Ruby versions on a single machine extremely important to a large group of people. Toss in alternate implementations like JRuby and Rubinius and it's almost indispensable.

The Ruby dev team tends to prefer encouraging the community to implement competing solutions rather than adopting any of them as official. Ruby devs in general tend to be members of the "there is more than one way to do it" camp.

There isn't really much of a "war". The different version managers came to an agreement on certain things like the format of .ruby-version files, so at this point it's a personal choice akin to which text editor you use.