The age isn't the only explanation for the low data rate- it's not simply low tech.

GPS relies on receiving extremely weak signals, often with a very limited receive antenna. This means there's a real trade off- you can send a limited amount of data over a given communication channel. Increasing the data rate might require increasing the transmit power / antenna gain of the GPS satellites, or requiring bigger / better receive antennas on GPS receivers.

Here are the parameters involved: https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...