Because although we did try to use MSTest for TeamMentor UnitTesting, it lacked a couple key features (namely the ability to define generic types in the Class Attribute).
Here is Michael’s commit that shows a simple NUnit test with multiple Browser invocations (note the TestFixture attributes):
Here is the replace of MSTest with NUnit:
And with the ReSharper extension installed, these ‘browser specific’ tests can be executed individually (i.e. run each set of tests on a specific browser)
See Run Selenium tests in multiple browsers one after another from C# NUnit (StackOverflow) for more details