These networks tend to be mostly WiFi based because the hardware is cheap enough that homegrown "low-tech" alternatives (like FSO or custom radio protocols) are more expensive.
I remember that ~15 years ago building these sort of networks involved various experiments with hooking up various home-grown radio modems or gutted laser pointers to serial ports, but today the technology of choice is either WiFi or commercial microwave PTP links operating in unlicensed spectrum (which are order of magnitude more expensive than WiFi, but have significantly better performance, also unlicensed spectrum usable for such links is not available in every country).
Perhaps "low-cost" is a better adjective. Wi-Fi hardware and knowledge are ubiquitous — the article says the buying Wi-Fi hardware is cheaper than rolling your own.
Also, I'd imagine if the people setting up the network leave, others will have an easier time maintaining if the network uses a very popular link layer.
I remember that ~15 years ago building these sort of networks involved various experiments with hooking up various home-grown radio modems or gutted laser pointers to serial ports, but today the technology of choice is either WiFi or commercial microwave PTP links operating in unlicensed spectrum (which are order of magnitude more expensive than WiFi, but have significantly better performance, also unlicensed spectrum usable for such links is not available in every country).