Why didn't the Eight Tails' horns grow back?
It comes down to the basic fundamentals.

The bottom half of the 8 Tailed Beast is an octopus, but the top is more bull like.

Bulls horns are like bone, unlike tentacles made of easily dividable cells, the horns of an animal do not "grow from the top but from the bottom upwards" they are like finger nails, the horns may eventually "grow" but not "heal".