Like many things in business its all about finding the right balance between risk and reward.
I am assuming that you dont want to pay a basic salary and commission because there is a risk that youll employ someone who doesnt sell anything and youll have lost out financially. You dont want to take the risk.
The same is true for salespeople.
If I were employed by you on a commission only basis and despite my best efforts your product doesnt sell - then I will have lost out financially.
So, in order for you to tempt me to work for you commission only the reward would have to be substantially higher to compensate me for the addition risk of potentially having no income. This is why you find fewer commission only sales jobs because employers are not prepared to offer high enough rewards to tempt sales people into taking that risk.
So by offering a reasonable basic salary plus a healthy commission the risk is shared and everyones happy.
Probably.