There is nothing special about 2.4 to 2.5 GHz except that it is a frequency band permitted for this purpose. It is EXACTLY the same band used for WiFi (although WiFi is also allowed to use the 5-6GHz band as well. Lower frequencies would penetrate food better but the magnetrons would be larger and much more expensive. Solid state power amplifiers may soon be economic for food heating and would allow for a wider range of frequencies if the regulators allow it.
John