Dragging your foot on a skateboard leaves your other foot planted on the board, meaning the board is vulnerable to catching on any crack or edge while carrying most your weight, which will send you tumbling.

You can drag your foot exactly the same way to lose speed on skates by dragging one perpendicular to the direction of travel, while your other foot is still perfectly capable of jumping you over any cracks or edges since the skates are strapped to your feet.

I'll admit it's easier to never go too fast for comfort on a skateboard, by simply not riding the skateboard; you get off and walk on shoes. Which is a problem for rollerblades. But that's kind of irrelevant beginner territory isn't it?

Once you're competent enough to be riding either at speeds above a run, the skateboard has significant speed governing challenges, as do rollerblades.

I street skated for years and never became competent enough to use power-slides for governing speed on anything but the most ideal uniform asphalt surfaces. To propose that as some kind of accessible braking method strikes me as disingenuous at best. For most, attempting that'll be a spectacular prelude to a broken clavicle.