Why do street names not have apostrophes?
Street names don't have apostrophes because the agencies in charge of geographic naming don't want to show the possession of a street.
In all honesty, though, nobody really knows.
In the US, all municipalities use guidelines set by state boards or commissions that regulate geographic names. These states in turn look towards the federal government for guidance. And on this federal level, the use of apostrophes has been discouraged since 1890. On the FAQ of the US Board on Geographic Names, they state that when place names are in the possessive form, the apostrophe is removed, although the "s" is allowed.
So, why, exactly, are apostrophes not seen in places like Harpers Ferry or Pikes Peak? The truth is that nobody really knows - not even the agency itself. "The Board’s archives contain no indication of the reason for this policy", they say on their FAQ. The probable explanation is that they don't want to show possession for natural features.
As for street signs, the federal government don't get involved, as those are left up to the local agencies, unless they are asked for an opinion. So, the local agencies are free to choose whether they want an apostrophe or not. However, they usually follow the guidelines of the state agencies, which in turn follow the federal government.
By the way, this is the article that I used to sort this quirky and somewhat confusing debacle out. Check it out if you have time. Definitely worth a read.