Why are electric heaters limited to 1,500 watts?

Because anything over that would exceed the safe amp rating of most wiring and would present a fire hazard.

Make sure you are aware of other fire hazards.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.