This is total speculation, but... I'm guessing that it also has something to do with the one boat per team rule. If all teams (defender and challenger) are limited to a single boat, how do you test? Back in the monohull days, most of the teams (especially the defender) ran multi-boat programs. Allowing the defender into the challenger round robins would reduce the cost for the defender by not having to build/test/maintain a second platform.

It also makes for better media, one more boat on the water to race.