I think the question was more about the "micro" part than the "SIM" part. It is a pretty new format, with the iPad 3G pretty much being the first device to use it I believe, so it wouldn't be too hard to believe that there wasn't enough supply to include them with all iPad 3Gs.
But yeah, from what I've read, they've been amassing these things for quite some time so that there would be plenty to include with iPads, and likely the new iPhones as well when they come out this summer.