Because parrying a liquid doesn't help.
spell to parry all ballistic projectiles
I would imagine that the spell applies an equal and opposite force to any projectile. When you apply a force back at a bullet (which is solid) it will stop the velocity along with damaging the bullet such as crushing it. At worst you'll end up with bullet dust on your shoulders (anything larger would likely fall off).
However applying a force back at a liquid projectile will cause it to splash (imagine a water balloon breaking) and not entirely stop it (less effective than hitting a solid object). The resulting smaller water droplets should have lower velocity but are equally wet. These smaller droplets are still moving so this process repeats until they are either too small for the spell to track or are moving slow enough that the spell won't apply a parry force back. Either way the water will eventually soak in.
This spell would protect against melee as well as projectiles but must have some kind of minimum size/speed threshold in order for the robe getting wet to be possible (otherwise the water would sit on top of the robe like a rain jacket). This also means that the robe's spell can't be used as a gas mask because gas is smaller and slower than liquid would be.
Real life acid rain isn't acidic enough to dissolve cloth from 1 exposure. It takes many instances over a long time to damage statues etc. But stronger acid would indeed dissolve the robe (and the wearer).
Because the robe doesn't cover the entire body
When you say "robe" I don't think of a dry suit. 580
Presumably the wearer's head, hands, and maybe feet are unprotected. The unprotected parts can easily get wet. Have you ever been wearing a coat and gotten snow down your back? Then you know that an unprotected head means water can get in and soak any part of your body.
see battlemages getting wet
It is entirely possible for a battlemage to be soaked yet the robe remain dry from the spell (as long as the spell also works from the inside out). Given this interpretation the spell has no need for a lower threshold. Even if the spell repels every single atom (including air) the wearer would still be able to breath (assuming he has no face mask with the same spell).
Unless the enemy attack completely overwhelms the battlemage in terms of magic strength
I read that to be a maximum threshold. The spell can only deliver a maximum amount of force and any projectile with more momentum will not be completely stopped. Or perhaps "overwhelming in terms of magic strength" is talking about an enchanted missile (for which rain is not) with an overwhelming amount of magic. The projectile could have a spell that prevents losing momentum in which case whichever spell is stronger would win. For an overwhelming number of projectiles see <PERSON>'s excellent answer.
I believe every option I've given for "the spell applies force" meets all requirements of the description. 537