Because it is reliable, cheap and moerse flexible. Also because they set off the biggest change in internet in South Africa and continuously doing so. Also, that data is unshapped.
Since I am not interested in any of those, but just stream shows from Twit.tv and the GFQnetwork, which is a whole lot better experience on Afrihost than on Plugg at the moment. Since all uncapped broadband providers are in a race to the bottom, the quality of the experience has gotten worst. Luckily, capped accounts have gotten a lot cheaper. So for the same money I can get a large capped account that offer unshapped data.
As for Plugg promise of a better uncapped experience, not sure that is true anymore. Having to switch between capped and uncapped depending on my usage need is certainly not ideal. Also, having a star rating dropped in minutes, then having to wait days for it to recover is also not a great experience.
Plus the whole less than stellar transparency as to how these threshold work is also terrible.