.aspx are active server pages built for Internet explorer. It's not the mac. It's the fact that Microsoft intentionally creates architectures that won't play nicely with others.
It's MS way of locking you into windows, IE, and their crappy system. Anyone who wants to argue this, is welcome to do so, but in this case it's simply the truth.
I hate that my company insists on building company wide web based software on this architecture, but when you have computer science majors who haven't been given better training in more cross-platform friendly languages, that's what you end up with.