We don't fully understand the tadpole - we haven't studied it, learned exactly what mental actions result in what effects, etc. We're feeling our way through usings its powers for our own benefit. And feeling your way through a thing, using your intuition: that's definitely Wisdom. At least that's my take on it. [...]
Does a starting sorcerer use wisdom instead of charisma because he's just figuring things out, feeling his way through how to use his powers?
How about a wizard's apprentice?
While I appreciate trying to come up with reasons why, sometimes I think it's better to just say it should probably be based on Intelligence.
A wizard's apprentice uses Int because Wizards cast spells by studying them, i.e., learning them. They have only studied and understand (intellectually) basic magic: cantrips or maybe 1st level spells. So it makes perfect sense that wizard apprentices use Intelligence.
Sorcerer could be a wisdom caster--growing in power as they become more in-tune with themselves--but in addition to feeling their way through their powers, a sorcerer is also channeling their inherent power into usable spells through their willpower. Expressing personal power is the domain of Charisma, which can be roughly summarizes as attractiveness (not just physical) + force of personality/force of will. This aspect of Charisma has been played down in 5e, so it's partially a legacy thing of earlier editions when the Sorcerer class was created.
Charisma measures your ability to interact effectively with others. It includes such factors as confidence and eloquence, and it can represent a charming or commanding Personality.
Commanding Personality is listed last and is and optional measure of Charisma.
Charisma measures a character’s force of personality, persuasiveness, personal magnetism, ability to lead, and physical attractiveness. This ability represents actual strength of personality, not merely how one is perceived by others in a social setting
Force of personality is first, a defining characteristic of Charisma. It also specifies "actual strength of personality," whereas in 5e Charisma is more defined relative to others.
