Campagnolo dropouts use 10mm x 26 TPI, mixed metric and imperial threading. SunTour and just about everybody else uses 10mm x 1mm. Also, there's a slight difference in the angle, crest and roots.While they're close, all these differences mean they're not compatible. Installing the SunTour has reformed the threads, damaging them in the process. Ideally you'd want to run a 10mm x 26TPI tap through the dropouts to clean things up before installing the Campagnolo derailleur.