Because you still have to tune the efi no matter what TB you run...A/F for idle is different than A/F anywhere else, and the transition can be tricky depending on the cam and the TB linkage and number of openings. A single isn't linear, more of an exponetial curve, but very predictable, a 4-hole progressive will not be anything even close and can really be a pain to tune with the IAC.

Singles are touchy in full tune mode. They can be like an on off switch, but a 4 link will like the harder hit better. Delaying it mechanically causes other issues too, and you can never really get the throttle response of a single out of a progressive 4 barrel.

For street, you just detune it a little with the VE vs TPS and the timing curve, and it makes it very smooth and not touchy at all. With XFI, you just use a switch that allows you to flip between programs on the fly with no computer.

Besides, as mentioned, four barrel progressive will actually be tougher to set-up, especially with a radical cam.