Cause the time it takes to make one, test it, and have it be "meh" by 1 person is about 8 months of time.
Unless I'm just completely ignorant of how this process works (which is possible), none or at least very little of that time is Relic's. There are already hundreds of possible maps you could use on the workshop with a little quality checking. Besides, every map is going to take someone's time, yet we got four 2v2 maps, plus several more 1v1 and 2v2s the last time they added maps. We can't even get one 4v4? Come on.