User blog comment:MisterWoodhouse/The first specifics on the Unified Community Platform/@comment-36453439-20191019145130/@comment-24155515-20191028140151

Probably the same reason Super Mario 64 DS is not a port of Super Mario 64 but instead a complete rebuild.

Sometimes coding standards have just changed too much in the interim and it's smarter from a code perspective to build something else from the ground up instead of applying glue to the thing you already have and hoping it holds.