Lower frequency signals travel further, and with less loss fo quality, and are subject to less interference.
Think of a car with heavy sound in it, blasting away some tunes, driving towards you. As the car approaches, the first thing, and the loudest thing you will hear is the bass, which is a low frequency signal. As the car gets closer, you start to hear the higher frequency sounds, from the Tweeters, and also vocals. After the car passes you, and drives away, the high frequency sounds dissappear, but you can still hear the bass pumping away...
Similar concept with 900mhz vs 21000mhz frequency bands for mobile broadband. The lower frequence signals travel longer distances, and can penetrate through walls and into buildings much easier, in the same way that pumping bass from a car does.