Why do paging systems need to provide low data rate?

Due to its low data rate, the signal BW is only around 5 to 10kHz. The sensitivity of a typical pager can comes to -130dBm. Meanwhile, since paging service is normally one-way, the transmitter can be very high in power level. They are typical around 100 to 500W and build on mountain high. So that's why the coverage is large.