Each radio on the tower is capable of 21mbits.. lets say there is 4 radios each covering 90 degrees and your in Plikkiesfontein with your phone connected to one radio. You now have 21mbits, now look around is there anyone for say 5KM's in the same 90 degrees arc as you with 3G ?
Now for every person doing data take the 21mbits and start dividing.
Feel free to subtract more from the total due to radio re-transmits if everyone does not have ideal signal.
That's the first part of the problem, now there's 4 people in Plikkiesfontein, each on their own antenna, going 21mbits, that's great but there's no servers in Plikkiesfontein. Oops. Plikkiesfontein only has a 32mbit radio backhaul (start dividing). Ok, now Plikkiesfontein's 32mbit has made it to Vodacom Midrand, along with the rest of the countries other 4gbit of demand. Great, there's 10Gbit of fibre running to Mtunzini and from there we hop onto the submarine cable. But oh no, there's only a STM-16 there (2.5gbits). Again, start dividing.
That pretty much sums it up. Think of the 21/42 mbit tower as your 100mbit network cable in your pc. The 100mbit part doesn't last long till your exposed to slower pieces of the pie.
EDIT: I don't actually remember Vodacom's international link capacities those numbers are just there for examples.