Mostly because the warm water has more energy to feed to the atmosphere than does the land. To expand on the first response, when warm moist air is lifted, it cools to a point where the vater vapor condenses into liquid water droplets that form clouds and precipitation.
Latent heat is released in the condensation process that adds energy to the growing storm.
Hurricanes only form over warm tropical waters and usually during late summer and early fall when the water is at its warmest. The warm moist air is the fuel that feeds the hurricane. When a hurricane reaches land it weakens considerably. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.