No it's exactly because he's not a great player.
Guardiola has never had a problem playing youth. His record shows he'll do it and he even gave Garcia a few games last year. Roberts has a whole pre-season in front of the boss to edge his way into his plans and if he hasn't done enough to earn a bit of faith then it's a shame but is what it is. Not every player becomes world class and not every player develops the technique and intelligence to work in a Guardiola system.
Not that I'm writing Roberts off, nor defending our quite frankly awful youth promotion policy, just pointing out that 95% of the game at Roberts position is about mentality. If he's decided he won't get into the City first team because his path is blocked then he's exactly right and should leave.