They have flashing lights and intense sounds, they are colorful, and pulling a lever is all the skill you need to play them. Even without the reward, it is easy to understand why people love them.
They are, after all, the simplest to pay, the most addicting, and if you end up being lucky - extremely rewarding. Slots have grown to become THE casino game, with slot machines all around every traditional casino, and there are always players at them.