Why was the decision made by GF that Slowpoke should be Psychic type?
It's mind-themed.
The same reason they didn't make Psyduck psychic type
I imagine it's the same logic that lead to them making the move Amnesia.
Slowpoke and Slowbro are actually so intelligent that they seem dumb to us.
The poison injected into Slowking's head by Shellder didn't make it smarter, it brought it down to human-level intelligence, which is comprehensible, thus creating the illusion that Slowking is smarter than its relatives.
>>32672241
i like this headcanon