Continuity is the reason. You cannot update the final scene without forcing the gap between the previous scene and that one even wider apart. The sad fact of the matter is the best they can do is the present and I think we already know what the present is since we live in it. As it stands, even with the upgrades still pretty much kept it in the 20th century. To properly upgrade it would involve all the scenes thus probably eliminating the first scene which is pretty important to the story, about progress made. If there is a need to go beyond that then a different attraction should be made. Maybe call it Horizons or something like that, but, it should stay were it's at because it does show progress from the beginning of the century to the end of it. Disney and everyone else has wisely chosen not to attempt to indicate what they think the future will be like. Two reasons: The first is that, at best, by the time they are able to physically upgrade it was already be obsolete and Second, nobody knows or can even guess on the future. Even Horizons attempt at it, although commendable, has not come close to what is actually happening. Do colonies live in outer space? (don't say Space Station, we all know that isn't a colony), Do we have undersea communities? Not unless we grow gills. How about the hover type agricultural methods, those are pretty common right?
What it would take to upgrade CoP to anything current would completely alter the meaning and the purpose of it to begin with. The idea that the last scene originally could somewhat depict the future didn't work out either which is why it was changed, but, now it would, like I said, create to much of a gap and become confusing. Going from the 50s to the next century would not be a good jump. Keeping it as a historical Disney classic, is the only thing that makes sense anymore, so if we want to keep it around, that is where is has to stay. They have tried more then once to shut it down and I'm sure the pencil pushers would like nothing better then to be able to justify leveling it. And that is something that no one should find to be a positive.
