Nintendo has been giving certain enemies so little attention. In New Super Mario Bros. Wii, the Hammer Bro only appeared in World 1-3, while only two Cheep Chomps were present. Of course, the Cheep Chomps were programed to give the player three 1-up Mushrooms when defeated, which can easily explain their rarity.
My own mod for New Super Mario Bros. Wii (which was made in 2013) does give certain enemies more course appearances, but it was removed, due to plagiarism issues with the courses; it's almost impossible to be original with your New Super Mario Bros. Wii courses now.