I believe the answer to this question, like many decisions at Microsoft, is more legal than technical.
Basically, before any major action is taken at MS, it must get passed the question, "If we do this, how likely is it that someone will sue us?"
Now, for MSFT to be sued over Classic ASP, the plaintiff would have to show that MSFT's actions prevented them from doing business (which here we'll assume that involves running a Classic ASP website). To reach that level, we'd need an IIS version N which cannot run Classic ASP at all, with IIS version N-1 no longer supported.
Verson N-1 of OS related apps tend to be supported 5 to 8 years after the release of Version N. On the other hand, maintaining minimal support of Classic ASP in IIS version N & N+1 etc should be trivial, and avoids the potential of lawsuit entirely.