They rely on it because there's no real good alternative for data. Europe uses TETRA [0] (the equivalent of the North American P25 but with better data support among other things) for critical communications but still needs a TETRA + 3G/LTE hybrid for large data transfers (above the hundreds of Kbps order speeds TETRA offers).

[0] https://en.wikipedia.org/wiki/Terrestrial_Trunked_Radio