There is no "why" -- it happened and was not selected against; it may have been adaptive and that is a kind of why, I guess. I think it would be better to be clear about this in science communication.
Of course there is a why. It happend and not only was it not selected against, presence of a tail was selected against. Meaning it was, or came along with a beneficial adaptation. That benefit is why it took over our gene pool and why we don't have tails now.
> But while the new study explains the “how” of tail loss in humans and great apes, the “why” of it is still an open question.
Title is still clickbait.
If it’s not selected against, it wouldn’t spread at the cost of the tail gene.
Somehow, somewhere along the genome line, non-tailers overtook tailers.