The problem is that the premise of this post is incorrect. While yes, Ratlings do make excellent snipers, there isn't much that keeps a Weapon Specialist from being one either.
Aptitudes, check. Both the Weapon Specialist/Ratling have what is needed.
Racial Bonuses: While the Ratling's smaller size aids in not being detected, it also makes them significantly more fragile. The Weapon Specialist is able to remain quite a bit more survivable.
Skills/Talents: Minimal differences; a Weapon Specialist has all the needed aptitudes to get the skills and talents a Ratling has using his 300 xp extra.
Gear: Only major item is a chameoline cloak. Which is useful, but the Ratling needs for survivability. GM's with even a bit of common sense will realize that a scarce weapon (e.g. a sniper rifle/long las) is certainly an acceptable piece of starting gear for a Weapon Specialist, and should allow them to take it (given they otherwise could take up to a very rare regimental favorite basic weapon). Also, if the regimental favorite is a sniper rifle/long las, well, problem solved.
Comrade Advances: trickier, but comparable. The ability to use the comrade to aim, while useful, only adds a minor bit of superiority to a sniper's economy of actions. They already can aim themselves.
The ability to make the shot unavoidable is probably the most powerful ability, but I would argue it contends with the ability to make the weapon pinning. An unavoidable attack is plainly damage; something everyone deals. Pinning on the other hand messes with the targets economy of actions. It keeps the party safe while forcing them to keep their heads down.
Therefore, Sniper != Ratling in OW.