The change in policy and practice is all about the money and cost-cutting.
The major airline alliances' policies had long been that access to alliance airline lounges had only one time-limit: the full day of ticketed departure as long as the alliance-airline-operated lounge had normal opening hours.
The major alliances' airlines don't really care about crowded lounges beyond what it means for their revenue and income. In that regard the airlines want to have space in their lounges to allow in more guests who mean more revenue and income from lounge operations. The recent shifts to 3 hour limits are about cycling in more guests into the airline lounges to get more money and cut costs per admitted guest into the airline lounges. They don't care if you have to put up with a packed lounge and you and a travel companion can’t find a seat together in the lounge and leave earlier because of that, as that is what the airlines want for financial purposes.