The principle seems to be that broken wands can't be repaired. It's testament to the enormous power of the Elder Wand that it can perform feats of magic that are beyond the norm.
Ron was having far worse problems. He had patched up his wand with
some borrowed Spellotape, but it seemed to be damaged beyond repair - HP: CoS
‘No,’ whispered Ollivander. ‘I am sorry, very sorry, but a wand that has suffered this degree of damage cannot be repaired by any means that I know of.’
Harry had been braced to hear it, but it was a blow nevertheless. He took the wand halves back and replaced them in the pouch around his neck.
Ollivander stared at the place where the shattered wand had vanished, and did not look away until Harry had taken from his pocket the two wands he had brought from the Malfoys’. - HP: DH
He rummaged in the pouch hung around his neck, and pulled out the two
halves of holly still, just, connected by the finest thread of phoenix
feather. Hermione had said that they could not be repaired, that the
damage was too severe. All he knew was that if this did not work,
He laid the broken wand upon the Headmaster’s desk, touched it with
the very tip of the Elder Wand and said, ‘Reparo.’
As his wand resealed, red sparks flew out of its end. Harry knew that
he had succeeded. He picked up the holly and phoenix wand, and felt a
sudden warmth in his fingers, as though wand and hand were rejoicing
at their reunion. - HP: DH
As Au101 pointed out in his comment above, it's theoretically possible that Dumbledore (who possessed the Elder Wand) could have repaired Ron's wand. This opens up a second round of questions; Did Dumbledore not repair the wand because he didn't want to give away that he had the Elder wand? Was there a reason Ron didn't ask Dumbledore to repair his wand?
Unfortunately, we don't know the answer to either of these.