A toothbrush is designed primarily for cleaning teeth: its bristles work the toothpaste in and act mechanically on the enamel and the spaces between the teeth. When you run a brush over your tongue, it partly loosens the coating but does not always remove it effectively — some of the coating remains or is spread around the surface.
A scraper works differently: its curved working part acts like a spatula — it catches the coating and physically lifts it off the surface of the tongue in a single pass from the back to the tip. The shape of the SATTVA scraper is designed to cover a large part of the tongue's surface in one stroke, which makes cleaning more thorough than with a brush.
In practice, many people who switch to a scraper notice that their mouth feels distinctly cleaner afterwards. This does not mean a brush is useless for the tongue — each tool simply has its own job. A scraper and a toothbrush complement each other well in a morning routine.