All those DAir's LeoN tried doing against Dabuz even though he adapted against them really costed him the third game, despite it being pretty close.
I can try to give my best answer based on my analysis of the set, but from what I've seen, the two reasons Dabuz chose
over
are:
A. Rosalina's UAir has much more juggle potential than Olimar, so Dabuz used Bowser's big body and his lack of landing options to his advantage during some points of the game; Bowser has DAir, which is a very scary move to get hit by, but it's also a very committal move, having an atrocious 30+ frames of landing lag (to put things in perspective,
's and
's DAirs have 24 frames and 22 frames of landing lag respectively); LeoN started going real trigger happy with the DAirs at one point in the set, and it ended up costing him a game.
B. Rosalina's ledge trapping, combined with Bowser's lack of get-up options, puts him at a huge disadvantage anytime Dabuz threw LeoN offstage. This basically happened during Dabuz's set against Elegant at Low Tier City: Just like
, every answer Bowser tried to scrounge up at the ledge, Rosalina would always have an answer for him, whether it was a grab or Luma's USmash (or insert any other nutty Luma attacks here). Bonus points to Bowser's fairly linear recovery, which Dabuz was able to punish thanks to Rosalina's/Luma's effective disjoints.
Funny how after he lost to Dabuz, LeoN continued his massacre once more, swiftly 3-0ing Jw and Suarez in a row to get to losers, lol.