I guess it has a historical (and technical) reason, at least here in Eastern Europe.
Fixed line telecom companies were among the first to offer Internet access over their fixed lines, initially using dial-up modems (33.6 kbps), then using ADSL over the same fixed lines (starting with 512 kbps). Then VDSL came with speeds up to tens of Mbps, over the same copper line (but shorter).
On the other hand cable TV companies started to offer Internet access over their coax cables (still kind of "fixed lines") using cable modems, then they moved to fiber optic cables, GPON, etc. All these are fixed lines.
Mobile Internet access came later, at least here, in Eastern Europe.
But it was probably the first kind of mobile access in countries without fixed line telecom or cable TV providers networks. In those countries mobile Internet access may be the only broadband connection known.
