Easy to measure and easy to budget for as directly linked to company income. And since a low basic & high OTE is also the market expectation, paying a flat rate either means shelling out a lot more money (because you're paying salespeople their expected earnings regardless of performance) or attracting/retaining weaker salespeople (because it represents less income for high performing salespeople and more income for low-performing sales people)

Certain types of high intensity sales roles lend themselves towards people who are highly motivated by opportunities for incremental commission increases too.