Because it's more profitable to sell larger accounts, even if fewer of them, than to sell many cheap accounts. One enterprise user at $100,000/year brings in the same revenue as 1,667 "hobby developers" at $5/mo, but requires significantly less babysitting.
Flip side is that you're less diversified and if you lose an enterprise client you feel it hard. There's more stability with lots of small users if you can make it work.
