Look at the map of the frequency of thunderstorms on the Ground:
Frequency distribution of thunderstorms over the land surface and oceans. The dark places on the map correspond to the lowest frequency (not more than 0.1 thunderstorms per km2 per year, lightest - more than 50).
On the map clearly shows that over land thunderstorms are more frequent than over seas. Why?
So the cloud is low, in the air under it should be a sufficient number of ions. Air, consisting only of molecules of nitrogen and oxygen, does not contain ions, and it is very difficult to ionize even in an electric field. But if in the air a lot of foreign particles such as dust and ions too much. Ions are formed when the motion of particles in the air just as electrified by friction together a variety of materials. It is obvious that dust in the air is much greater over land than over the oceans. Because of that storm and thunder over land more often.