Slow Data rate of GPS data
The low data rate comes from the need to transmit the data reliably over a noisy channel with a low power transmitter.

A GPS receiver has to be capable of picking out a signal under conditions that other radio systems would just call "total loss of signal."
In a typical two-way radio (analog police radio and the like) the signal is usually unusable if the received signal strength gets below -120dBm.  Digital signals usually get lost long before you get down that low.

GPS receivers work in ranges where a good signal is -155dBW.  In order to fish the data out of the noise, they use a faster signal (1023 million "chips" per second for the civilian codes) that is then modulated with the actual data.  You modulate the data on the chip signal, and modulate that onto the transmit frequency.  You use correlation on the chip signal to recover it, and demodulate the data from that.

The end result works like a very narrow band filter that only lets through data at a low rate, but very clearly.  

The chip modulation scheme also has the advantage that you can transmit a bunch of them on a single carrier frequency, but be able to separate them cleanly on the receive side.  This is known as Code Domain Multiple Access (CDMA) and is the reason that the US GPS system only needs a single transmit/receive frequency despite having some 30 satellites.  Glonass uses 15 carrier frequencies (each used twice) to cover the globe with 30 satellites.  It makes the transmitters and the receivers just that bit more complicated.

I would suggest reading the Wikipedia GPS article.  It includes a lot of details I've glossed over in trying to keep this short.