Every method of boarding will require training the flight crew, ground crews at the airport. A non-trivial expense to change over from A to B. And then back.
Departure times are measured down to the minute. Miss your time and much activity downstream is affected - pax connections, gate use at destination airport, flights downstream.
And so on. I'm sure that they do use A/B testing or something like that ... but they do a whole lot of simulatin' first.