Fewer of these things apply if you make it a doubleheader.
You still have the added payroll and decreased consumer base. Remember you already paid those people to show up to work once. Even if they didn't work their full shift, they did show up, do their pre-work and break-down if they're concessions staff, etc. And depending on who they are/what job they do/your contract with the food services vendor, you might have to pay them to hang around in between games.
Yes, I believe you can make a case that a traditional doubleheader is better business than split. For the reasons above plus the fact that you get a huge bump in attendance. People like to see two games for the price of one.
That gap in between games puts a lot of people off because it makes it *such* a long day. And the lines for the second game have been really awful which also puts people off. Nobody wants to wait in line 30 minutes to get in. If you take a quick break (like they do in the minors) and let people stay rather than making them leave for 2 hours, you can sell them stuff while they're waiting and make money.