Yeah Slaking has great offensive stats with good bulk and good speed. His Base Stats are the highest out of all non-legendaries at 670, so it would be a good question, why isn't he a Psuedo-Legendary?
The answer is in his ability. Truant. This means he can only make one move every 2 turns which destroys all his usability since an opponent can combine protect + an attacking move to completely destroy Slaking.
Additionally Pseudo-legendary is used to refer to any Pokémon that has a three-stage evolution line, 1,250,000 experience at level 100, and a base stat total of exactly 600 - Slaking is above