It is easier to crowdsource once the main outline is in place, it structures a vague problem into a set of more specific subtasks that can (more or less) easily be distributed across many people.
Like in this case: Deconstructing an existing proof can work very well with crowdsourcing, since a proof contains a relatively small set of specific claims, and each "crowd" contributor can focus on one particular issue.
But this works because the crowd now has a common focus point and task list created by the paper being published. It's not clear to me how (or whether) the process could be reversed, that the same crowd and effort could be coordinated to cowrite an original paper with an alternative proof.