Trademark inherently can’t be generic. It has to refer to something distinct.

For example, apples have nothing to do with computers, so the name makes you think of the exact brand Apple computers. But if you tried to trademark your apple company Apple, that would be taking away the ability of competitors to describe the product they’re selling. Which is not ok.

So, when your brand name becomes the common word to describe a product, you have to stop your competitors from using your name to advertise their products in the first place. Otherwise, your brand name doesn’t just refer to your products, so it’s not distinct. You’ve become Apple apples.