In the case of Stack Overflow, I think the reason for the data dumps was two-fold: one of the original founders (who left long ago) came across as at least idealistic and wanting to do the right thing. The other was pragmatic and most likely always thinking about the money angle. However, the other founder likely also saw the value of the data dumps from a PR standpoint which was quite valuable as they were initially trying to replace expertsexchange.com that paywalled most of the content. IIRC, they discussed the data dumps in the early days of their podcast.

Now that there's big money to be made from machine learning (both the models and the data they are trained on), they've likely decided 'screw it' on the PR value of the data dumps and would rather get some of that sweet, sweet machine learning money.