I understand there's more nostalgia behind PS2 since there were more PS2 users. But still, I find it odd that the original Xbox isn't going up any. With multiplatform titles, Xbox versions are usually the best versions to get. They usually perform the best and have the best graphics, so that's the version you usually want. Plus a number of titles are backwards compatible on both XBox 360 and XBox One.
As mentioned somewhere in the previous comments, 1st gen xbox games really didn't age well. Gamecube games on the other hand have all aged beautifully unlike its N64 counterparts, basically all hit Nintendo titles on the GCN like Metroid, Mario, Zelda still hold up amazingly today and are exceedingly popular even with younger children still. Just look on twitch and see all the 13 year olds streaming Mario sunshine or Mario 64.
Same goes for PS2, I mean some of the PS2 exclusives that came out were just incredible. The ratchet, sly fox, jak series, final fantasy and other hit JRPGs.
With the GCN and PS2 so stacked with its amazing library, there was no way xbox with it's extremely limited offering of halo, ninja gaiden, mortal kombat, GTA (which btw is also all released on the PS2) could've competed. That is to say, the xbox was not a failure, fact quite the opposite. But the point is it really doesn't have the same potential in present and future collection value in comparison to the two other 6th gen consoles.
That being said, with the exception of maybe very very few select xbox titles, I don't see the value of 1st gen xbox collections shooting up like GCN or PS2...like ever.