Median energy density in waves is too low in most places. You need way too large machines to extract useful amounts of power. The few times you get sufficiently powerful waves they tend to rip your equipment to bits.
Wave energy is one of those ideas which seem really obvious from a distance, so the fact that project after project fails does not seem to dissuade anyone. They were obviously just doing it wrong.
I really hope that I am wrong and this turns out to be a great success, but I am not holding my breath.