I'm not sure by now the language's merits are the number one reason Fortran is used, but rather the amount of proven and super fast libraries for number crunching and array/matrix manipulation such as LAPACK.
It's the only language that has quality, actively-developed compilers, excellent support for multidimensional arrays and can be very effectively optimized. The libraries are a bonus, but support for proper multidimensional arrays (i.e. not arrays of pointers to arrays) in other widely-used high-performance languages it either crippled or non-existent. It's a bit baffling that something better hasn't come along, but I think the wealth of Fortran libraries and amount of work on compiling it would make it hard for a new language to get traction.