Planes don't use blue liquid anymore. They use a vacuum system that uses barely any water and as a result, the urinal vs toilet is a wash (pun intended). Urinal would take up more space too with no benefit.

On the older planes like 752's etc. they use the blue liquid still. I believe the liquid is recycled though through a filtration system. At this point I speculate it isn't worth the refit and cost of trying to change.