Wild Mass Guessing for the original Super Mario Bros..
The Brocks of the Mario & Luigi games are an unintentional result of Bowser turning Toads into blocks here.
Bowser had turned most of the Toads into completely inanimate blocks (i.e. the ones you spend the game breaking and running across). However, some of them got stuck in an in-between form that was mobile and humanoid, but still blockish. Upon Peach being rescued, a good deal probably figured that there was no real disadvantage to being in this new form, so they decided to stay that way.