There is no historical reason why slur should be any politer. Its first meaning, as recorded in the OED, was a thin sort of mud (slurry came from the same family). The 17th-century verb is defined as "to stain, smirch, sully" - not much difference between this and smear. However, there was some ambivalence about a slur. It was a question of whether the mud stuck. When Victorians talked, as they sometimes did, about "a disgraceful slur" they implied it shouldn't stick, but when they said "a slur and a disgrace" they probably meant it should. Incidentally this has nothing to do with the word police witnesses use for the speech of drunken suspects, which is differently derived, and began by meaning a gliding or sliding, once an Elizabethan dancing term, later a word for a smudge on a printer's proof; people used to accuse each other of "slurring over the facts".
Anyway, there's no doubt now that smears are more malicious than slurs. Meanwhile if the charge really is justified there's always stain. Mud can be wiped off, but a stain on the character (like a blot on the escutcheon) doesn't come out.