Another good reason is that on a big server execution speed is not so much an issue as the connection speed anyway. Most of the time is spent sending and receiving data, not number crunching. And actually in certain web services which do a lot of computations, the hard crunching is probably run as a compiled program.
Plus interpreted languages don’t need compiling (which on a large project can take time), thus it’s more suited for the typically agile development of web solutions.