A luxury? They had the luxury to make an entire extra episode for DOOM II and to edit the secret levels, but didn't have the luxury to make the games run in widescreen? Yeah, right.
Upset future customer.
You've obviously never been at the business end of modifying rendering code that was written a decade and a half ago. Making a new level wads and modifying some game image assets is simple compared...very little can actually go wrong doing that.
Modifying how the entire game renders? well you'd have to do full testing on the entire game to make sure it all still worked properly and even then you might miss stuff. 90's game development isnt known for it's strong programming standards.
EDIT: I'm not saying they shouldn't have done it, I'm just saying I understand why they haven't.