Because there are other low level dungeons that are more interesting. And by "other low level dungeons", I mean "literally anything else that isn't Thousand Maws".
I'm perfectly fine with doing low level duties, I really like some of them. Haukke is nice, Brayflox NM is fun, Cutter's is decent if a bit long, Dzmeal is great, and Stone Vigil is probably my second favorite dungeon in the game, behind pre-nerf Pharos. Sastasha, on the other hand, just feels terribly boring. The bosses are completely uninteresting, the setting is bland, you have almost no abilities because lollevelsync and it feels like it drags on for way too long. Some of this is to be expected, obviously, since it's the first dungeon in the game, but even Tam-Tara and Copperbell are less tedious. Plus, the roulette seems to derive some sick pleasure from giving it to me multiple days in a row.
I don't quit out of dungeons very often. Thousand Maws is the only one where I will 100%-immediately-without-hesitation drop out because everything about it is absolutely terrible. But Sastasha is a pretty close second and frankly sometimes I'm just not in the mood.
If they'd split up the low level roulette into two separate queues (one for 15-32 dungeons and another for 33-49 dungeons) or allow for a roulette blacklist (ie. choose two or three duties you refuse to play and it won't pick you for them) it would alleviate a lot of this.