Most likely because they are 64 bit only. Sampletank 3 has always been 64 bit. Kontakt was recently changed to 64 bit only but the 32 bit version is still available but has to be installed manually.
It would be nice especially considering the future for 32 bit looks bleak. Of course Metro runs fine on Windows 64 bit. The problem is that Apple forces one to change their programming language when switching to 64 bit.