There’s an extra detail that might be important, which is the tournament settings.
Why are the 19x19 ones less popular?
Well for one the live tournament setting is Swiss, so not only will it take longer than the Blitz tournaments and smaller board ones by definition, who even knows how many rounds there will be ? There’s discussion in another thread.
Blitz 19x19 would be expected to be more popular, by comparison
- It’s faster than live
- It’s elimination so you play till you get knocked out, and you roughly guess how many rounds are left.
9x9 and 13x13 will be shorter too just because of the board size, but again even though there’s a 13x13 Blitz, it’s still Swiss… so while the games will be shorter than 19x19, how many hours of games are you signing up to?
In general how many hours of games are you signing up to with 13x13 Blitz Swiss or 19x19 live Swiss? Is it somewhat inconvenient to have your next round start randomly when the other games are finished? Wouldn’t it be better to just use automatch or a custom challenge where you have more control? 
I think not knowing the answers to these questions just put me off the Swiss format altogether for ASTs, also possibly not getting a break between games if I’m the slowest player.