Is it popular? Or merely heavily used?
As far as I can tell the team that I used to work with use C# merely because that is the language promoted by Microsoft and Microsoft is the principal supplier of IT infrastructure to the company. I don't think anyone ever said "Let's use C# because I like it." They usually said something like: "Ugh, got to get away from VB6, what can we use that won't need explaining to the technically illiterate bosses?"