Why is it that satellite TV requires aiming a satellite, but satellite radio doesn't?

The freq at which these are transmitted. As some one earlier said TV signals need a higher bandwidth to carry all the information. Therefore need a very high frequency carrier. When the frequency is higher you need a eyesight contact. On the other hand the bandwidth required by an audio transmission is very low ( some 20 KHz) SO a low freq like 1 MHz can carry the audio signals. They don't need eye to eye contact. They disperse very easily and propagate.

Satellite radio also requires aiming if You live out in the Boonies,but if You live in certain urban locations You are most likely getting the signal from a repeater.

Both XM and Sirius have repeater networks,but they don't issue the locations to the public.

Do you mean satellite dish? TV signals take up more "space" with audio and video, while radio is only audio and it is more easily picked up by the receiving unit.