I think you are reading too much into this. They probably simply have a project related to CRM, and have decided to implement it using .NET technologies.
.NET, like any development framework, is not tied to any specific field of problems (though it is a popular choice for typical "business/enterprise development"). But if a project uses .NET, job ads for it will ask for .NET experience.
As to the other technologies mentioned (SAP, Siebel or MS Dynamics): I don't know these in detail, but both Siebel (now owned by Oracle) and SAP use their own frameworks, and have good interfaces to Java, so I'd expect projects using them to require Java skills rather than .NET (though interfacing them with .NET is certainly possible). MS Dynamics, unsurprisingly, has close ties to .NET, so there I would expect ads asking for .NET experience.
Ultimately, all systems support standardized interfaces (Web services or similar), so you can make them work with any technology; it's just that each system has some technologies it is more closely associated with.