This may be a stupid question to ask but I’ve never been able to get a straight answer anywhere. Was there a reason that newer source games like Portal 2 were once mountable and their props usable but this was then disabled?

Yeah, this is something that has confused a lot of people. When they were first mounted they were using a different system Since then Valve changed them over to SteamPipe – which is a completely different system. I didn’t have time to add support for the new system – but it is high up on my huge todo list.