Nowadays cellular networks offer very fast connection and have very high coverage of the territory and in most countries areas out of coverage are uncommon. So, I would assume a drone that use cellular network would solve many of the poor signal issues for FPV and telemetry in long range scenario compared to direct Wi-Fi protocols that with low transmission power regulation are very limited unless you are in open areas with little interferences.
For this reason, I'm wondering if cellular network is just not much explored for this use case only for the need of a support provider and incur costs, or there are also other technical reasons that I missed.