Because they ran the numbers. It wouldn't justify the valuation their investors demand, and it would be a distraction. Ads are their only hope. Draw your own conclusions.
If you're a b2b customer paying $$ or $$$$, you would expect an SLA, customer support, etc. (think AWS). All that is expensive, and if the revenue would not be in the ballpark of ads then Twitter won't do it now. They need to find their cash cow before they can explore smaller markets.