Because it's not worth the support and complaints. A business account paying $$$ is more likely to see the benefit and the money goes out from preallocated budget.
The analogy in my head is the most excellent little bespoke bun shop I go to every week with my wife which is awesome. Compared to buns made for the masses in the supermarket.
Maybe there is a market for craft software as in craft beer :-)
Not that smaller customers are bad, but the ROI isn't as good. I've found that they're much more relaxed and easy to work with compared to working with other startups. The other startups I've worked with on deals often have shorter timelines and the person you're talking to is either a founder or early employee and has way more stake in this transaction that a manager at a larger company does.