We believe it's due to the land absorbing heat and causing strong convection. On the other hand, water can store a lot more heat, and releases it slowly. Lightning patterns also vary from one season to the next. You see lightning activity truly following the sun. You see a similar pattern in the southern hemisphere, but not so pronounced because there isn't as much land outside the tropics.
Scientists discuss biology, safety, and statistics of lightning strikes.
Poster papers on hurricanes and tornadoes summarized. Thunderstorms generate elusive and mysterious sprites. Preview of the 11th International Conference on Atmospheric Electricity. Variations show up even from one year to the next. It's tightly coupled with the dynamics and physics of the storm.