I guess gamestudio is developed with VS 2005. This redistributable package was of course earlier included to the windows installation/update than the one of VS 2008.

I don't know why the new redistributable isn't installed on so many pcs... Only microsoft knows why it is how it is.
What I know is, that the new Gamestudio installer installs the VS 2008 redistributable package for GED. So I see no reason why a game creator can't include it into his installer.

In general it's the same thing like the ogg filter. You also have to include the installer to make sure it is installed on all systems.