Electrical energy initially tugs the coaster's cars to their maximum height, at which point the electrical energy converts to gravitational potential energy as the cars teeter at the brink. In theory, this sort of sequence could go on forever, energy switching forms continuously. It would make for one very long roller coaster ride. But reality, alas, is different. Along the way, energy is constantly being diverted or lost.
Friction gets a lot of the blame.
Resistance between the cars' wheels and the coaster rails or between the riders and surrounding air bleeds away some energy, primarily as waste heat. Other energy transfers occur as well. Energy is released in the sounds of creaking wood and clanking steel, in the vibrations of the framework and ground. This energy, like friction heat, doesn't flow back and forth between potential and kinetic. It's simply lost to the environment.