Season 1 has been on my "Buy when it becomes one of those $14.99 sets at Best Buy/Target in a year or two" list. I love the Cos but there are plenty of sets I want more.
It might have helped if they'd bothered to promote it. Hard for people to buy something that they don't even know exists. Some people don't even remember the original series much less know about the DVD release.
TV and radio advertising for starters. Magazine and newspaper ads have also proven effective for DVD sales. Another common method is during the preview reel in movie theatres (which worked for the Seinfeld discs).