>>58029490
The super tight cartridge capacity is probably a myth as we know there's lots of unused content left in the ROM, but the reason they didn't restore more of Kanto in Crystal is just that it wasn't their priority at that point. They already accommodated for the missing parts in various ways, like spreading the old safari rares through other areas and destroying Cinnabar, and I think they wanted to focus on new stuff rather than backtracking on all those changes. I guess it would have been nice to have more Kanto dungeons to find the old legendaries in, but it's clearly an intentional design decision that a few rare old mons are exclusive the the time capsule, they could have always made the birds roamers if they wanted them to be available.