The cheaper your board, the more porous it is. Ink can dry into these tiny crevices, making it hard to erase. For instance, this is very common if you have a melamine whiteboard.
Whiteboards become rough and scratched with repeated use; essentially, they experience wear and tear. This makes it a lot harder to erase and clean.
Another reason is that the materials in the board react with the ink, causing it to set into the board.
To summarise, when you get a whiteboard, it’s smooth and slick because of the finish. Over time, that wears down, making it harder to erase!