My hunch: baseball used to depend on selling tickets. A double header was a way to attract more fans -- "two games for the price of one!" A few companies bought season tickets, as a treat for customers. Boss and I took a couple of programmers from Morgan Stanley to Yankee Stadium around 1988. Saw Kirby Puckett.
It seems that more fans buy season tickets, so a team wants one game per day. I miss the old-fashioned 1pm double header and the "twi-night" that started around 4 or 5.