Consider the medium of air as defined by the use of radio frequency. What made some of the early standards so slow compared to today's 802.11n 300Mbps and higher bandwidth

Earliest standards were dependent on a single frequency/channel to both send and receive. This shared medium creates the same problem as half-duplex coax cable. Because receivers had to wait for the signal before sending a response, this reduced the overall bandwidth.

Other factors affect wireless signal propagation, too, including RF interference, antenna choice, and obstacles such as walls, trees, and even weather (precipitation, for example).