Too old for your rubbish..
It was due to merchandise, that's why Axew didn't evolve, even though it wanted to evolve all the way to its final form, the only known Pokémon who wanted to evolve all the way to its final form and succeeded was Ash's Caterpie.
Actually, as far as Axew goes, I'm fairly sure the only reason ever stated by the animation staff for it not evolving, was that it'd be awkward to explain after having Axew travelling outside it's ball and in Iris's hair for so long.
It wasn't like Fraxure could do the same, so they would've either had to somehow explain why it wasn't allowed outside it's ball anymore, or draw it awkwardly walking alongside the group all the time.
They should've just had it in a ball to start with, but after keeping him out, they made the right choice by not evolving him.. because you just know the story behind Fraxure suddenly being in its ball would be complete rubbish.