Because it's an anime? The age demographic of the audience doesn't really change the term applied to it. It's an anime and that means we call it an anime.
An anime is literally a Japanese cartoon anyway, so by definiton, only the cultural differences split the two terms. Western culture has simply applied the word "anime" to Japanese animation (the word being an abbreviation of "animation" in Japanese) perhaps as a means to separate it from other animation styles and to have a way to refer to it specifically. This is useful, as the animation style referred to as "anime" has differentiated itself from other forms of cartoon through its varied styles and traditions. For this reason, it may be viewed upon as entirely separate from "standard" western cartoons. This is why anime tends to be segregated and categorised apart from other forms of animation. If you were about to watch an anime, you'd have a different expectation than if you were watching the cartoons you mentioned.
To be clear, you wouldn't be wrong in saying the Pokemon anime is a cartoon, because it is. It's simply more correct to say it's an anime since that directly specifies it's a Japanese animation, and also suggests the aforementioned styles and traditions that are applied to Japanese animation.
