One big reason and probably the most technically compelling one is that they are smaller diameter, and if you have a really skinny rim you can't afford a large hole for the valve. In addition the valve head is self cleaning so you don't need a cap and it's tradition. I'm talking road bikes here not Mtn bikes.
All these would come up with a Google search BTW
The majority of good wheelsets, tubeless or otherwise, are Presta. Presta is bike specific while Schrader is automotive. Generally, cheap bikes and wheels come with Schrader valves, probably because that's what non-cyclists are used to.
As already mentioned the smaller hole fits smaller rims. It also makes a stronger wheel. Presta is easier to adjust air pressure and with the lock nut installed, filling a flat tire is much easier than Schrader because the stem doesn't get pressed into the wheel when the pump nozzle is pressed down on the stem.
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