I also find it strange given that most roguelikes -- well, many at least -- ask you do to 100 floors down, and then 100 back up.
Maybe it's because in this game you are quite specifically tantalizing the player with what is going to happen at the end. So, when some players are getting to the 20 days left mark they are more anxious to see the end than they would be with another title in which they don't know exactly what's coming up.
Also, I definitely wouldn't call this a short game -- not at 50 days anyway. At least, for a rogueLITE. For example, FTL can be played and completed quite easily in an hour. Even in Dredmor, I think it plays a lot shorter than this game, although that's usually because your often dead before you get very far.
If this game were a flat 50 days, I'd probably say it was a little long for my taste. For some reason when I get to the 15 or 10 turns left mark I start getting a little fatigued. Exactly why this is the case, I haven't fully analyzed. However, this game DOES come with a very nice way to shorten a campaign so I don't think you should really be getting any criticisms at all on this front. I just don't think new players are fully understanding how those Robot Research Center missions can be used to steamroller through a campaign. Well, those two reviewers at least definitely didn't make use of the ability. Maybe those two reviewers are not indicative of what future reviewers might complain about, but I think if you could emphasize the role that Robot Research Centers can play in the length of a campaign then that would at least soften any potential criticism in this area.