Jedi Dark Knight said:
I always had the idea that Skywalker was a pretty uncommon name in that universe, maybe because of the connotation of the "one that will bring balance to the Force", imagine if someone said "a Smith will bring balance to this country(US)" the search would be mute, but if you are going with a pretty uncommon last name is sort of like less candidates to fullfill that prophecy (which by the way the prophecy was known only to Jedi and Sith)

By that same token, it would be more difficult to track down the individual if it was indeed a common name. If the prophecy spelled out the name explicitly, the powers-that-be would just be on the lookout for that one person. Regardless, prophecies by nature are vague and wouldn't name names in the first place.

But this is all hypothetical at best, and just for our own personal edification in knowing Ben's and Vader's motives (and not necessarily Lucas's limitations - which are great in number). :)