There's many reasons for this, one that's simpler to explain is relative performance.

If sales person A is working a lot harder/smarter/adaptive to get more clients that sales person B. It may lower his morale to see B get the same compensation as him. This affects sales people more than other functions because the output of a sales person is more or less quantified by the revenue they are responsible for generating (exact metric would be different of course).

As always, you get exactly what you are measuring.

I've experienced sales people who oversold more or less everything. They weren't measured on whether or not what they sold could be delivered.

Instead, product teams had to run around as if they were on fire, because suddenly the CEO got involved and made it a "company wide priority".

I hate KPI's / commission structures, and I have never seen it working. Not where the work required more than half a brain cell.

I guess my great hatred for KPI's is that they are often vanity metrics. What you want to measure is net contribution to the bottom line, and that is near well impossible. So we end up measuring what we can, and Goodheart's law [0] being what it is, get absolutely no value out of it

[0] - https://en.wikipedia.org/wiki/Goodhart%27s_law

Bonus info: I've had work, where I did not have KPI's, but where I could destroy the KPI's for other teams, and sometimes involuntarily did.

To motivate them to sell more.

why not use the same tactics for developers and others ? is it because it easy to measure it for sales but not for developers ?

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.

A) First and foremost, yes, that's why.

B) I think the assumption is that 100% of salespeople are motivated almost entirely by greed, whereas it's less clear that this is as simple with people in general.