Why do microwave ovens use radiation with such long wavelength?

I wonder if a bunch of microwave questions that appeared here recently is related to an enlightening (as usual) recent what if by xkcd.

The radiation used in microwave ovens is not resonant for water molecules and industrial microwave ovens use 915 MHz (probably, because larger cavities can produce more power). The frequency of 2.45 GHz is chosen because it falls in one of the bands not reserved for communication purposes. According to Wikipedia, the next available band would be at 5.8 GHz. A powerful magnetron working at this frequency is feasible but way too expensive for a household appliance. If such magnetrons ever get cheaper, it is likely that there will be a luxury microwave marketed for its uniform heating profile.