Because sans-serif is easier to read. It even says that on the page you link to:

"Due to the basic constraint of screen resolution—typically 100 pixels per inch or less—the serifs in some fonts can be difficult to discern on screen."

Because you read Wikipedia on a screen and serif fonts on screen are harder to read? That's my opinion, I bet others prefer serif fonts even on screen, but I feel that they make it a lot harder to read.