I admit I don't follow the Windows world very closely, and I'm no developer, but Wikipedia says .NET got started in 2002, by which time Solaris was already on its way out.
I'm not sure what you see in .NET that you think killed Solaris/SPARC, or where you see the overlap (where a lack of .NET could have presented an opportunity for Solaris), but from my point of view Solaris was an also-ran by 2002, even though I continued to administer it until after the Oracle acquisition.