Second, and most important of all, is the pixel size of the 1st gen back sprites. They all take place within a 56 x 56 usable pixel area. However, every pixel is magnified by 2, causing the effective area to actually be 28 x 28. Not a lot of space for detail, is it? The only way to avoid having hideously blown-up images like these
would be to make higher-res versions of them. This act doesn't even include the colorization process, which would take place later. But what would the 100+ hour effort lead to? Just a bunch of sprites of 1st gen Pokémon that resemble their most grotesque iterations (JP Red/Green). Even more, the back sprites in the 3rd Gen games can have an effective area of 64 x 64 pixels, so some of the 1st Gen back sprites would look smaller than they should be.
Third, the back sprites in the 3rd Gen games are already retro/nostalgic. If you hadn't noticed by now, the first 251 Pokémon are using their exact same back sprites from the 2nd Gen games (which were based off of the front sprites in Gold), only colorized and occasionally extended. This means that the back sprites do not match with any of the front sprites in any of the 3rd Gen games (save for 3rd Gen Pokémon, of course, which have only one primary front sprite to speak of).
Any effort to create "old-school" back sprites would only result in complete and utter chaos. Those involved with such a project would most likely be wasting their time. It's just best to let some things die easily.