User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-151.229.88.58-20201221111153/@comment-1038387-20201229171419

Because Legacy is unupdatable.

It's 8 year old core code with over a million lines of custom code, and also some that have overwritten the core. That's a bad idea, in hindsight. To update that, all those custom codes need to be checked, separated, updated, et cetera. Every time they do an update. And then fix everything that breaks. Which is why it didn't happen for 8 years. This also blocks new features from being added, and makes fixes take longer. It's an incredibly inefficient behemoth held together by duct tape and magic.

With UCP, the core was restored to pretty much vanilla, and the custom code was compartmentalized or, in some cases where it could not be salvaged, discarded and/or replaced with alternatives. That makes updating in the future easier.