No reason for the change was disclosed. Perhaps the localizers thought "impact" better reflected his abilities? Maybe they thought "pile" carried some kind of connotation? Who knows – it's up for speculation.
Beyond the whole name thing, the website also brings us some new information about Impact/Pile Man himself. According to his profile, Impact/Pile Man's body "consists of three pile driver robots who call themselves the Impact Brothers: Kui-ichiro, Kui-jiro, and Kui-saburo." That's pretty neat, yeah? You can read more about the big guy here.
While you're at it, check out some voice samples: Japanese version here, English version here.
Source: Official Rockman 11 website