I doubt weaponry is the reason so few WWI games have been made. My guess is that it has more to do with the fact that it, unlike the second world war, can't really be portrayed as an epic battle between good and evil (ie. the game industry's favourite narrative).
Instead it was more about innocent men being slaughtered in the thousand in what was - or at least appears to be - a meaningless conflict. In other words, in a WWII game you know you're killing Germans because they are nazis and thus evil, while in a WWI game you'd be shooting Germans mostly because they'd otherwise shoot you.
This, of course, is also the reason why games like Call of Duty, Medal of Honor and the like are almost always played from the Allied perspective. It's probably also the reason why later conflicts like the Korean War and Vietnam hasn't spawned nearly as many games, despite featuring similar or even more advanced weapons.
BTW, regarding civil war games, Sid Meier has made a couple some years back, and, in a much less serious vein, there's also Infogrames' evergreen classic
North & South.
