ASP does not support all SSI directives?

I read from the IIS documentation about the #flastmod Server-Side
Include (SSI) directive that "Because you cannot map a file name
extension to more than one executable, you cannot use thisdirective in
ASP files. ASP files are already mapped to Asp.dll and must stay that
way."

Then shouldn't ASP support all SSI directives? Why doesn't it delegate
SSI directives to ssinc.dll?