traingang

Its both. Or all 3 I guess.

1. Non-US countries do a much better job of grade separating their high speed rail for this reason (among others). Introducing new level crossings is illegal even in the but in the US they don't care, most of brightline's route is covered in street level crosssings
2. And also, the level crossings that brightline uses are dogshit, they're basically the same as all low speed rail crossings in the USA, a single bar for each direction of traffic (covering only the lanes going that direction, not the whole wide-ass roadway), depending on street layout they can be trivial to drive around, and often placed high enough that small vehicles can reasonably drive under it with minimal damage (it will hit their windshield and deflect up out of the way. I don't think people are doing this intentionally but it's not a serious barrier is my point).
3. and finally, american drivers are pretty dogshit and often think they can beat the train (american freight rail is so dogshit that if you get stuck at a crossing you might be stuck there for 2 mins or 10 mins or in rare cases 30+ mins, so people not expecting an 80-120mph train feel incentivized to cut it close to escape being stuck at the crossing). It does sort of ironically "sneak up" on people. obviously there's plentiful warning signs but the ability to see/hear it coming is much less than with a freight train.

edit actually

4. they aren't permitted to sound the horn for much of the route, which exacerbates the above
5. there isn't even fencing along much of the corridor so they also hit pedestrians crossing outside of street crossings

only 13% of the fatalities are people in cars, most are pedestrians or cyclists. The first fatality was a young bipolar woman hit by a test train before it even opened, and the company insists on calling most of the deaths "suspected suicides" with no real evidence.