It's because the populace has a different understanding of what qualifies as offensive. Blackface has a thorough link to minstrel shows and its offensive content in America. Blackface here is uniquely linked to Zwarte Piet, which is not considered offensive in its modern form by Flemish/Dutch people.
What exactly is the modern day representation of Pete?
And blackface has the same link in Holland. Pete was created for that same purpose, to demonize and create a negative view of blacks. They might have changed his story over the years, but the link is still there. It would e a different case if they created him without the racial intent, then you could say that blackface in holland has no link to negative and racist views on black people, but that isn't the case. Even if the story has changed.