(continuing) Then, when the company began creating new sites outside of the Trilogy through Area 51, the limit was set to 1 since those new sites were quite a bit smaller than the Trilogy, and so it effectively became the default since the thresholds were never changed. As to why you found that the change was made in 2015, my guess is that someone was experimenting with changing it at the time but ultimately reverted their experiments.

I've done some research and based on early revisions of the bumping FAQ on Meta.SE and this post from then-lead Jeff Atwood, it seems that the limit of 2 at a time instead of 1 dates back to when the feature was introduced in 2009, not 2015. Best I can tell, the default was 2 on all sites (which at the time consisted only of the Trilogy - Stack Overflow, Super User, and Server Fault). Then it was increased to 4 on SO due to its large size.