i would think that for one, if speedball was set up it would obviously not be air bunkers as they would have water in them. Also, speedball fields are smaller and having people on that field most of the day it would get quite slushy and nasty if there was any amount of snow on the ground. Woodsball also attracts more people because they dress in more layers with camo and such, where as speedball you are wearing a lot less clothing as to be more nimble and have your body feel less restricted by clothing. Woodsball fields are a lot more open and people arent running sprinting as much as you would in speedball therefore you would have less chance of slipping and sliding around while trying to move up from bunker to bunker.
That all being said, there are still speedball fields open in the winter, just maybe not in your area. Its really up to the field owner as to how he has the fields setup during that season. I would just assume there will be more attraction to people who play woodsball during the winter season than speedball.