Windows CardSpace != WCS
Windows CardSapce is unique , thus even by it's name. "Windows CardSapce™" is the official name of CardSpace and needs to be mentioned only once in the begining of an article, after that it can be referenced as CardSpace.
Unlike it's colleges to the .NET 3.0 Framework the WF , WCF and WPF - it's not labeled as a Foundation , niether can it be short handed as WCS, why ? for starters WCS seems to stand for Windows Color System. If you think about it , sometimes it's good to be unique and shortning the name would do no justice to the name.
Not only that, but I have a feeling that unlike the others it has an unofficial mascot - the raccoon. Fact Vittorio Bertocci both in his book The cover of our book "Understanding Windows CardSpace" and in his post added pictures to "Credentials vs. Identity; Authentication vs.... what?" use the racoon as a symbol.
In the end, because CardSpace is a new technology and a relatively new concept for many, Microsoft is trying to establish CardSpace in people's minds as an association with a safer online identity experience than users are currently exposed to. Therefore, using the name CardSpace will clearly help establish that.
... for more read Richard Turner's Windows CardSpace <> WCS!