Location and availability, also bandwidth. A carrier may offer unlimited 30mbps, but how much of it is available/usable in a given area? A carrier that provides a higher volume of cellular towers will probably be more likely to have less congested bandwidth and offer better connectivity at lower speeds, a little more expensive sure, though without the aggravation.
Using the map on the link, selecting country and carrier, one can see quite a disparity between some of the carriers.
https://www.nperf.com/en/map/TH/-/19345.AIS-Mobile/download?ll=18.76421779682828&lg=98.97891998291017&zoom=11