Because they wanna teach you things in order: you learn the radicals so that you can later learn the kanji and you learn the kanji so that you can next learn the vocab.
The point is, learning the vocab today or in 2 days is not a big deal at all (this is using reorder wisely). However, due to the fact that there’s no limitation on having to learn the vocab in order to level up, the reorder script can be abused and vocab lessons can be avoided completely. This leads to bad learning (using reorder unwisely).
So it depends on the user really (and their priorities: fooling themselves or actually learning). WK chose to not make this type of reordering available for all users. However, that action is still possible due to scripts. Which WK allows btw. It’s not like WK is preventing that from happening through scripts. They leave it to the users.
I’d say that the reorder can be dangerous in reviews (vs lessons) for more users though. So lately I recommend this Lessons reorder instead since it gives you no power to mess with reviews (my guide needs an update - I know :3):