The amount of integration between the system and the wifi modem, the frequencies that it can broadcast on, and the regulations for that part of the spectrum.
You'd be quite challenged to make your wifi modem connect to an access point a few miles away. Phones do it all the time. A phone may be broadcasting with as low as a milliwatt when near a transmitter to a few watts when further away ( https://en.wikipedia.org/wiki/Wireless_device_radiation_and_... ). Wifi has a much smaller range of acceptable broadcast power available (and at the most powerful end of acceptable is less than a phone).
You'd be quite challenged to make your wifi modem connect to an access point a few miles away. Phones do it all the time. A phone may be broadcasting with as low as a milliwatt when near a transmitter to a few watts when further away ( https://en.wikipedia.org/wiki/Wireless_device_radiation_and_... ). Wifi has a much smaller range of acceptable broadcast power available (and at the most powerful end of acceptable is less than a phone).