Why do hurricanes lose power once they reach land?
Because storms draw their power and strength from the water they form over. In shallow waters like the Gulf of Mexico, the surface temperatures tend to be warmer, which can cause the growth of superstorms like Hurricanes Katrina and Rita. We all remember the amazing amounts of damage these two storms were able to do, yet once they made landfall, they rapidly lost strength and fizzled out.
Think of the large bodies of water, especially warm water, as food for hurricanes - more evaporation, more precipitation, and more wind speed. Once on land there is nothing for them to continue to draw energy from, and the storm moves north and loses strength.