[i_are_smart]

Because one of the characteristics of McMansions is that they (poorly) copy the style elements of classic designs while ignoring the rules that governed those designs.

Modernist and postmodernist architecture isn't trying to imitate classical mansion architecture.

[gclaramunt]

But then, "copy the style elements of classic designs while ignoring the rules that governed those designs" will make it automatically bad, and IMHO many aesthetic trends started like that