Because when you are teaching art history it's impossible to cover all the movements and teach all the artists that participated in each one in a single semester unless you are doing an in-depth course on surrealism specifically. Every school of art usually has a set of "founding fathers" that were credited as being the first to work primarily or soley within that set of principals.

Dali is the poster boy for surrealism the same way that Monet is the poster boy for impressionism, Piccaso represents cubism, Warhol for popart, or Seurat is the first person when you think of pointilism.

He became famous because he stuck his nose in many popular projects. Dali also made forays into film, most notably in Un Chien Andalou, a short surrealist film made with Luis Bunuel which famously features the image of an eyeball being cut open by a straight razor. (The effect was mimicked using clever editing and a cow's eye.) He even did a famous scene for Hitchcock’s film, Spellbound. Less memorably, Dali contributed a dream sequence to the 1950 romantic comedy "Father of the Bride."

He stays famous because his art is very good and stands the tests of time.

Because his paintings make people think. Then people will have the need to talk with others about how they think about Dali and his works. Decades later now and his paintings still make us think. That is the reason.