900 Mhz I'm assuming a cell phone based system - very low speed, about 19.6 Kbs. Due to many cell towers with high power antennas and low bandwidth. Think of a filling a bucket of water with a pressure washer - you could shoot that water a pretty good distance but the flow of water is only a couple cups a minute. Cell phones were designed to pass audio which is very low bandwidth - 8 Kbs voice is understandable. Cell towers use very expensive transmitters, antennas and towers - tens of thousands of dollars.

802.11 is 2.4 Ghz at 11 Mb/sec - high speed. The Wi-Fi infrastructure isn't set up a cellular network - they're normally dozens of individual networks - the coffee shop, a local school, private ones in homes. These things are regulated to have very low power and use small antennas. I have a couple of external antennas with some gain that extend the range but still it's only a hundred yards. So the individual networks aren't designed like the cell towers to pass customers from sight to sight. Each time a Wi-Fi client crosses out of the coverage of an access point it has to find and renegoicate with the next site. At 120 miles an hour you'd be coming in and out of AP's faster then you could connect. Think of filling up that same bucket with the open end of a garden hose - you'll only get a couple three or four feet but you'll fill the bucket up a couple gallons a minute. AP's cost tens of dollars.

To clarify...

The wider the channel,  the more information can be transported through it.

You can have two radio systems that are virtually identical and operate on the same frequency,
but one is a voice radio and the other is a broadband data radio.    Just to make this easy, but not realistic,  say both of them are operating on a frequency of 500 MHz.

The voice radio is designed for a bandwidth of 25 KHz.  This means that the 500 MHz channel it's on actually covers all frequencies between 499.9875 MHz and 500.0125 MHz. ( +/- 12.5 KHz per channel, from the center frequency)

The broadband data radio may also operate at the same center frequency, but say it has a bandwidth of two megahertz. (2 MHz)    In this case,  it actually uses all frequencies between 499 and 501 MHz.   It uses more frequency space.

One thing to know:  The broader the bandwidth of the signal, the more power it needs to have the same effective range as a narrower bandwidth signal.      Amateur radio operators that operate with morse code only (CW operation)  use a signal that's so tiny that it has a bandwidth of just a few Hz, yet only a few watts is enough for most of their communications, even on a worldwide scope.

CJ