One of the problems with monster books is that it takes a certain amount of system mastery to design mechanically cool monsters, but you want to get the classics done ASAP (remember the flack Wizards got for not having the full complement of giants in the 4e MM1, for example). For example, look at 3.5e: mechanically, the monsters in MMs 3, 4, and 5 were way more interesting than the ones in the main MM. But all the common monsters were in the MM: goblins, gnolls, orcs, ogres, giants, dragons, demons, devils, a bunch of fey, and so on.
For example, MM5 had a cool devil with the ability to dominate people by grappling and swallowing them, and then spitting them back out. That's awesome. But since it was in MM5, no-one ever heard of the gulthir, and instead everyone uses bearded devils for low-/mid-level diabolic encounters. It also had variants on the badly designed ogre mage (elemental mage) that no-one ever used, and so on.