Whereas Super Metroid sticks around in the speedrunning scene because of its open nature, Super Mario Bros. sticks around for the exact opposite reason. Speedrunning Super Mario Bros. is a fairly linear experience. The best path through the game has been mapped out many times. The challenge this game offers speedrunners is the pursuit of perfection.
It's almost like an Olympic sprint in that fractions of a second often make the difference between a record-breaking run and one that does not, so speedrunners shave those precious moments off of their time whenever they can, keeping their momentum, making pixel-perfect jumps, and generally entering a supreme state of concentration and flow. It's cathartic to watch as well, especially since speedruns of the game generally hover around 5 minutes due to all of the warps.