A tail would be incredibly helpful for balance (tripod example), productivity, etc.
Having four legs and a tail would be more advantageous for balance than just having a tail (calfs for example; they can stand up on all fours an hour after birth, but their productivity in building a shelter is quite dismal). Actually, the ideal would be for humans to have four legs and a tail (for balance) and two arms with hands, fingers and opposable thumbs at the end of the arms (for productivity).
I read that it could have been the climate change from forest to desert that could have been the reasoning.
From the evidence it seems as if the great apes never had all those legs to improve balance. What a pity. It seems as if the great apes were badly designed.
What is the best current explanation? (other than natural selection/god just did it)
We have empirical evidence for the existence of natural selection. The evidence is also verifiable.
We still have none for the existence of a supernatural designer. Not empirical, neither verifiable.