Understanding how it works, whether through instinct (Wis) or study (Int), will not suddenly make the item operate.
Charisma is used because the idea behind it is that the user is *tricking* the item into "thinking" that the user meets the requirements to use it. Consider it a modified version of the Bluff skill that only applies to magical devices.
Using Int, you could figure out that a certain ring required a command word to activate, but that revelation doesn't impart the knowledge of what that command word is. You have to *trick* the item into "thinking" that you know the word.
Any spellcaster can use a wand if they are of appropriate level and have that spell on their spell list. But you can trick the wand into assuming that you meet these requirements.
Does that make sense to you now?