In practice players rarely change the gametype of a server.
The point also is, but I may be wrong, that there should be fundamentally no difference between standard votes and custom votes. So your wish about making it standard is like wishing the bit of code which implements it is written here instead of there, but finally that's the same. Custom vote possibilities are the same than ones you have with rcon access, almost any cvar can be accessed and any command can be issued. Imagine there is a new gametype which uses exotic cvars (freezetag, anything), developpers wouldn't have to mind about which vote can or cannot be issued, because custom votes is flexible, that's a little win. If the whole vote system can be configured then we could assume it's admins task and developpers could even deprecate the whole standard vote system. Of course for this to come true, custom vote system should be tweaked a bit and may need to be better integrated.
Like usual, a long debate emerging from such a small thing, heh