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The available spectrum for broadcasting is controlled by the local government and there are severe restrictions on use. Aside from that the range of a 315mhz is MUCH greater than that of a 2.4ghz system with physical objects nearby as the greater the frequency the more matter it will interact with.
No actually, you have it backwards. The lower frequencies will generally travel through materials better than higher ones, that's how Xray photography works the denser and object the more xrays will be absorbed. If xrays passed through the body without interaction then x-ray photography wouldn't work at all because nothing would be absorbed. That's a general statement though there are very different absorption rates of various objects at different frequencies this is how spectroscopy works.
Another reason for using high frequency X-rays for imaging bodies is that the resolution of the image is related to the wavelength of the electromagnetic wave and the higher the RF frequency, the shorter the wavelength. Objects with size approaching the wavelength of the wave become blurry.
To clarify, X-rays are the same type of electromagnetic wave as RF, just much higher frequency.
This is one reason magnetic resonance imaging has become popular. In a sufficiently dense magnetic field any object will reflect/absorb RF radiation at a much different range
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Another reason for the different frequencies other than the differences in range is that the signals do not interfere. i.e. if they transmitted at the same frequency the signals from one device would get picked up by another. This is also part of the reason for the government regulations that sceadwian referred to.
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