900 MHz and 1800 MHz are used for GPRS in the UK. The higher the frequency the more directional they become when propagated, which is why most cell sites have three or six antennae, to give 360 degree terrestrial coverage. To give you coverage in the air they'd need another two or three, pointing skywards, but since not many people fly, there's no money in doing so!
There are other reasons as well
