Actually there is a fork with GL support. It may be added to the official port eventually, I know that some of the devs are keen on seeing it included as an option eventually.
As for why it it does not have it already, well that's because Odamex is based off of a very old version of ZDOOM, and ZDOOM has always been software based.
As for me, I find that GL makes things look really nice, but it also brightens the hell out of everything when things should be dark. As an example, I have a few rooms and hall ways in my project that are very very dark. In these rooms are pulse lights that travel the length of the hall, or spins in a circle within the room (like DOOM 64). What this does is adds a little Alien-esque tension where in the player can only see the enemies when the light happens to dance across them. Add GL to the mix and you can see the enemies all the time, regardless if the light is on them or not. So basically what needs to be done is I would have to go back through my map(s) and adjust the lighting of every room and dramatically decrease the light level in hopes of restoring the intended effect...
...which is not exactly what I call fun.
