Legacy had a chasecamera, was coded in C, and wasn't a big C++ rewrite mess like ZDoom is. It also has (had?) the best network play capability of any port.
SRB2 actually started out in TeamTNT's BOOM for a very short time in 1998.
"Why don't you move it to ZDoom/GZDoom/DOSDoom/JDoom, etc. etc?"
Because you can't just 'move' SRB2 to another port. It's so totally different from Doom you just can't do a plug and play deal. Look at JTE's failed attempt to move SRB2 to ZDoom - you have to rewrite the entire game to do something like that. To move to another source port, it would be as much work as if I just wanted to code SRB2 using The Wizard Needs Food, Badly.
When making a game, if you expect to achieve progress, you have to set goals, choose the stuff you're going to go with, and stick with it. Otherwise you end up with something like Sonic: Project Mettrix (no offense to Stealth), that ends up with new rewritten engines all the time but nothing new in terms of game content.