I'm not sure a lot of fans are dying to hear YNM live
Yeah.
Let's say the band flat-out loves You Not Me, which would surprise me but isn't strictly impossible, I guess. The thing is that DT doesn't just play shows for their own sake -- they're cognizant of the various things they need to accomplish crafting a setlist. To oversimplify, suppose you could split the songs DT typically plays into three groups:
1) Super-popular songs. These are the ones
everyone at a DT show knows, probably because of media exposure of some kind, and most attendees like or love the song. The crowd response is usually enormously positive here. These songs are often recent, but not always. I'd throw Pull Me Under, Metropolis, the fan favorites from SFAM, Constant Motion, the Dark Eternal Night, and to a lesser extent OTBOA, The Enemy Inside, and The Looking Glass in this bucket.
2) Popular songs. You have to be a more devoted fan to know these, but they're well-liked if you have exposure to the band outside of Guitar Hero. Probably put the Count of Tuscany here. Maybe Surrounded. Breaking All Illusions. Trial of Tears. Maybe, uh, Blind Faith or the Glass Prison or something. I don't know. Crowd reaction to these should be more restrained but still positive.
3) Less popular songs. An interesting group: for a number of reasons (including limited familiarity among the audience with the song), crowd reaction is minimal. The first one that comes to mind here is Scarred, which was dropped from the CiM tour because the crowd would lose interest during the song -- no fault of Scarred's, because it's an excellent song, but it's sort of long and complex and goes in a lot of different directions very quickly, and so you're not likely to get into it if you're a casual fan hearing it for the first time. Maybe New Millennium should go here, along with a few of the more obscure songs from FII. Raise the Knife probably goes here for obvious reasons.
Maybe you don't agree with this grouping but it seems at least slightly accurate to me (though the divisions between the three groups are somewhat blurry). Anyway, there's a place in the setlist for all of these kinds of songs, but where I'm going with this is that
You Not Me doesn't belong in any of these groups. It's a song that a lot of DT fans aren't familiar with, and it's probably not entirely unfair to say that the majority of DT fans who know the song don't like it very much. It's not considered a hidden gem. It's not adored by a significant portion of the fanbase. It's usually only mentioned as a joke or as an example of the influence the studio had over the album's songwriting. There's no one clamoring for its return live (unlike, say, Space Dye Vest had). Playing You Not Me just wouldn't make any sense. It's not a crowd-pleaser. It's not well-liked. What would be the point?