There is no official reason for why the 3PO droids were designed like that, but they were intended for use in controlled/indoor environments presumably, since they are protocol droids.
It is also largely unnecessary. C3PO was (re)constructed on a desert planet and ran around for 10 years or so without any plating at all with no apparent issues.
At some point, however, C-3PO fell into disrepair, and his vital
components ended up in a junk pile on Tatooine. Anakin Skywalker, a
slave boy from the Tatooinian city of Mos Espa, collected scrap parts
and started rebuilding C-3PO so the droid would help his mother.
Although protocol droids were normally designed for light duty in
luxurious environments, Skywalker specially modified C-3PO so he could
withstand Tatooine's sand and heat. C-3PO served Anakin and his mother
Shmi by performing household chores. During his time with Skywalker
and Shmi, C-3PO's wiring was left exposed since Skywalker was unable
to outfit him with an outer covering.
C-3PO | Wookieepedia
So the exposure of his wiring is not a hinderance in any way.
Plus, any dirt gives them an excuse for an oil bath.