The main reason is the choice of signal encoding over the Ethernet cable. IEEE chose baseband signal encoding as opposed to carrier signal modulation, which, like WiFi, and even traditional radio, would have had much greater range. read more
Not sure why people worry about cable length and speed. Data travels at some fraction of the speed of light in copper cables. Does it take longer to go though a 300ft cable than a 10ft cable yes some tiny fraction of a millisecond. read more