When cross platform isn't an issue (and it often isn't), I will chose C# over Java for a project. Hands down.

I can accomplish more work in fewer lines of code with C#.

Reasons I prefer C#:

? Proper support for properties: public int foo { get; set; } vs getFoo() setFoo()

? Operator overloading: foo == "foo" works as expected.

? Generics work with primitive types: List vs. List

? DateTime and Timespan just work (mostly fix in Java 8, I think)

? Partial classes.

? Clean separation of UI generating code from codebehind logic in winforms and web apps.

? LINQ + Extension Methods + Lambda expressions + closures. This is the #1 reason right here. This was partially fixed in Java 8, but they still have a long way to go.

Because project requirements and .NET is just better than what Java offers. Java is great if you don't care about security or stability. C# is still better than Java regardless of what Java 8 does to close the gap. Because not everyone needs to have multiplatform code developed, and some people just prefer Microsoft's better tools over Oracle's crap.