It is for the same reason that I wish Safaricom make billions in profit this year. I am yet to come accross a provider who wants to sell Internet, and they seem to be doing so involuntarily.

Airtel is congested,  and their 3G still needs some work.

Orange insist modem users don't need one day unlimited Internet,  and appear shocked when you tether their one day unlimited on the phone,  which lately has had speeds capped such that it doesn't even feel like 3G.  Their fair usage policy is inconsistent, meaning you have no idea what you are paying for.  At ksh 40,  its a bargain, but I'd rather pay Safaricom ksh 200 and get Internet I can use.

ISPs are busy "investing into our own fibre networks to give more value to shareholders ". 

In short, with more than 50 registered ISPs, only 1 is effectively serving Kenya's 38 million citizens.