You’re backwards. They didn’t start caring about the opening weekend until the opening weekend became truly national. Up until the mid 80s opening weekend was just for big cities, then over the course of the next month or so it would move down the city sizes. Then the system changed to hit everywhere at once, many more prints, Chicago and Alamogordo getting first showings on the same day, and suddenly opening weekend really meant something.
Of course since then the number of movies they make jumped dramatically, so now all but the most profitable movies won’t even be in the first run theaters in a month, so opening weekend becomes even more important.
The two trends reinforced each other.