The main reason for me is the nice array notation, and many other design decisions that make writing and debugging scientific code easier. The fact that it is usually the best choice in terms of performance on the relevant tasks (array operations) does not hurt either :)
Honestly, I would not consider most the languages cited as real competitors for Fortran -- Java and Ruby are far, far behind in terms of both convenience and performance, while C++ is much too complex and tricky a language to recommend to anyone whose main job for the last few years has been anything other than daily programming in C++. Python with numpy could be an option though. I am personally not a huge fan of the language, but I know a number of people who use numpy regularly and seem quite happy with it.
Real competition I see is not from these, but from Matlab, R, and similar languages, that offer similar convenience, combined with many standard libraries. Luckily, it is usually possible to start a project in R or Matlab, and write performance-critical parts in Fortran later.
