Bostontrainguy (like the name btw, am from Boston myself), there seems to be a trend towards single-level/ high level boarding these days. Amtrak in Illinois is ordering the same type of cars Brightline is using, and Amtrak California is too since Nippon Sharyo dropped the ball on the new Superliner car design, so the money had to go somewhere. Caltrain is also planning to go high-level for compatibility with CHSR.
Why did Brightline go single level? I think the answer is design, they want to go 125mph, which takes certain equipment to do. The problem is that there are only 2 FRA-compliant bilevel cars on the continent that can go that fast, and they are both commuter cars, the MARC III and the Bombardier MultiLevel. Now they could use these, but Brightline has standards of service, and I don't think bumping your head on the ceiling (the Multilevel was built to fit in the NYC tunnels) lives up to those standards.
If Brightline needs more capacity, they can just lengthen the trains.
Personally, I don't want to walk up what amounts to multiple flights of stairs while walking through the train, so I'd much rather ride single level. High-level platforms and single-level cars are the most ADA friendly cars (think of the elderly who really can't do stairs unlike college-age me).