802.3 v/s 803.11

This gives a nice overview of the differences between  Ethernet and Wifi at a protocol level.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.456.9874&rep=rep1&type=pdf

• The crux of the problem is this :  “The CSMA/CD protocol is not used in a wireless environment due to the user has no capability to sense/listen to the channel for collision while sending the packet [12].“

• This necessitates things like Collision Avoidance techniques to be used for Wifi.  And that imposes limits on how fast you can transmit packets at a certain frequency band leading to slower speeds.