Do you mean Afrihost Mobile? If so, then what you're asking is "why is MTN's 3G performance slower than other Service Providers' 3G performance", because Afrihost uses MTN's 3G bandwith. It may just be that you are in an area where the MTN signal is less-than-ideal.
If you test a normal MTN sim card's 3G performance and compare it to your Afrihost's sim, and there is a big difference, then something is wrong and you need to contact Afrihost. They will do their best to sort it out. If the performance is the same, then unfortunately it is out of their hands as performance is area/signal dependent.
If you are asking why Afrihost ADSL is slower than 3G or LTE generally, then the answer is that they are two completely different technologies with different limits. Also, if you are on an uncapped package, then your package will be limited to the speed that you pay for (eg. 2mbit, 4mbit, 10mbit etc).
Edge/3G/H+/LTE etc are not specifically limited in speed by the ISP, and will perform as fast as they can at the given moment, depending on the signal and area that you are in at the time. In real-world scenarios for South Africa, an average / good LTE connection has been known to reach speeds of up to 20mbit (and more!) at times.
Hope this helps!