Western Dressage - 1 handed?

Very genuine question, not here for a raging battle.

I took a look at a majority of the pics taken at the WDAA World Show in Oklahoma. Why are the majority (if not all) of the horses that are ridden in curb bits (not snaffles) being ridden with 2 hands?

I was wondering why no one was riding with 1 hand (splits or romel reins)? Do you get dinged for riding 1 handed? Do people think that self carriage isn’t possible with 1 hand?

I’m a romel rein lover myself and I’m just dipping my toe into the WD world but I wanted to get some feedback on the bit/rein situation.

I saw some decent shanks on a few bits (who knows about the mouth pieces) but 2 hands. Was excited to see tons of snaffles and a few bitless bridles too!