Probably no particular reason other than a two-word last name is cumbersome and/or too ethnic for some tastes. If you're Van Schmuck, is your last-name initial V or S? If it's VanSchmuck, it's obviously V.
It's also possible that the change goes back to the anti-German sentiment of the Big Wars (especially the First), when many German-Americans changed the spelling or structure of their last names to seem less Teutonic. Even the Royals in the UK (of German extraction) changed theirs from Battenberg to Mountbatten, and Saxe-Gotha-Coburg to Windsor...
Pick one of the above or none. Your guess is as good as mine.
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