* quantlib: aside from (proprietary, very expensive, and damn slow) Matlab, no other language has a library of quant-related functionality that is so vast
* A lot of 3rd party libraries and APIs that do not have .NET, Python, Ruby, R (you name it) wrappers and you do not have time, expertise or resources to write them
* Like wglb mentioned, quants are obsessed with performance. Even aside from obvious things like high-frequency trading where you try to squeeze out every milliseconds. Let's say in middle office you run risk measurement calculation daily and it finishes in 9 hours for your portfolio. Well, if you happen to triple your portfolio (not unheard of in boom times), you cannot run it daily anymore, so a factor of 1.6 gets in your way here too.
* .NET programmers (on average, of course) tend to have less experience in dealing with algorithms and data structures
Comments
There are many reasons.
* quantlib: aside from (proprietary, very expensive, and damn slow) Matlab, no other language has a library of quant-related functionality that is so vast
* A lot of 3rd party libraries and APIs that do not have .NET, Python, Ruby, R (you name it) wrappers and you do not have time, expertise or resources to write them
* Like wglb mentioned, quants are obsessed with performance. Even aside from obvious things like high-frequency trading where you try to squeeze out every milliseconds. Let's say in middle office you run risk measurement calculation daily and it finishes in 9 hours for your portfolio. Well, if you happen to triple your portfolio (not unheard of in boom times), you cannot run it daily anymore, so a factor of 1.6 gets in your way here too.
* .NET programmers (on average, of course) tend to have less experience in dealing with algorithms and data structures