There is a very good reason for making the Archaeologist badge slightly less rewarding than Strunk & White badge. When a user edits a post, it is bumped back up to the top of the active queue so that other users will see it. The point is so that the community can review the edits as a quality control measure. For newer posts, this also gets the question more attention in general, which can help lead to answers, votes, and comments.
With older posts, the same thing happens, the edited posts are bumped back to the top for the same reasons. However, they may be less valuable, since they're more likely to have answers.
The goal of the Strunk & White and gold Copy Editor badges is to encourage editing, since editing is key to quality control. Now, if you just happen to stumble across an older post during a Google search while searching for a question you have, and you just happen to edit it to fix some major problems you see, then that's awesome! But encouraging people to go on a witch-hunt for such posts may not be the best thing for the main page.
Thus, the badge is there for people who happen to fix these posts as they see them, but the reward is not set so high that the editing focus is taken away from the newer material in need of more substantial edits.
Unlike some other badges, Archaeologist isn't synonymous with the Strunk & White or Copy Editor badges, or Enthusiast and Fanatic badges, or Announcer, Booster, and Publicist badges. This is probably why Archaeologist has some slightly higher threshold requirements than the silver and gold editing badges.
Update on now-deleted comment:
If you really want the badge, go for it :) A few avid users who really want the badge won't significantly clog up the system. It's when everyone does it that it becomes a real problem. Consider that badges are designed to encourage certain desired bahaviors, and the right mixture of reward/cost help Stack Exchange tune those behaviors so they're at exactly the right levels. – jmort253 yesterday
NOTE: This was really bad advice and can actually clog up the system. One of the reasons this badge is a little harder to get is because Stack Exchange doesn't want to see the main page cluttered with too many older questions. Since each edit has an effect on what appears on the homepage, edits to older posts should be clearly substantial and add significant value to the post.
I withdraw my recommendation to go on a "badge run" for the Archaeologist badge. Focus on the Strunk & White instead! :)