I think a lot of the charm of RCT2 comes from the specific assets in use and the degree of whimsy and joy they contain, more so than TTD's matter-of-fact trains and planes and such.
It's not like these folks are setting out to make a totally free-as-in-beer game that replaces the old one; they just want to be able to fix bugs and add features, and you can't easily do that with the original retail stuff.
https://en.m.wikipedia.org/wiki/Copyright_Term_Extension_Act
And the data files from the demo version do the trick too.
Contrast that to OpenTTD or OpenRA where they (AFAIK) re-implemented the engine while still having the same approach of re-using the main games assets.
Edit: taking a look at the source code, I don't think what you're saying is actually true. RollerCoaster Tycoon 2 was written entirely in Assembly but I cannot find any Assembly files in OpenRCT2. So unless they already finished porting everything, it seems incorrect.