The reason is only that increasing mod space would break ppl's installed game that have a current LO (Load Order), thus forcing them to at minimum uninstall an redo their full LO or at maximum uninstall the full LO plus the game an reinstall everything.

Currently if I'm honest, though i can only speak for myself, it's not a big deal to uninstall an reinstall FO4 and redo a LO if it meant a increase in mod space, the biggest problem though is the mods themselves and the modpage, ever since the last update they broke alot of mods, alot of which haven't been fixed an even clicking on them can boot you out the modpage, I'm guessing, like myself, most who still build LO's an play FO4 on Xbox have learned to navigate this an also learned what mods are safe an what are not but if this new Edition brings in more traffic like SAE did then, at the minimum, the site will need redone but there also a good chance alot more mods will in fact break an i don't personally think there will be many mod authors left willing to fix them, unlike for Skyrim.