I'm unconvinced by the "GB can be played on GBA" argument. Look at Playstation games, for example.
I think the bigger reason is that people just think of both GB greyscale games and GBA games as "Gameboy games" - they don't think of the games as "last generation" or old or anything like that. Another reason is that GB games hold up quite well - people tend to pay more for cartridges than for CDs for some reason.