Because most Fortune 500 employees by definition are code monkeys that won't touch FP concepts, just doing the typical CRUD applications full of design patterns.

Having FP concepts sneak in C# is easier to get them introduced in this world, than forcing this type of developers to use something like F#.

This why Erik Meijer decided the best way to bring FP to the enterprise was via Visual Basic.

http://research.microsoft.com/en-us/um/people/emeijer/papers...