Probably because being a Navy aircraft, it operates in a high humidity environment which is conducive to forming these types of clouds. Also, to photograph this, the jet needs to be going really fast, and typically very low--something you can't do freely over land, especially if you happen to slip into supersonic flight while doing the high-speed pass.

Over the ocean, there are no such restrictions.

Here's another link http://www.eng.vt.edu/fluids/msc/gallery/gall.htm for more cool fluid mechanics photos.