1) Time - it takes tiem to create, author and encode the extras.
2) Money - HD media is still a niche market so very few studios are likely to gain sufficient extra sales due to special features to offset the costs.
3) Importance - Is havving the feature in HD with lossless sound enough to satisy early adopters? For me it is.
Space is no realy the issue, as HDDVD has shown you can easily fit a 3 hour epic + extras on a single disc. But how studios make use of the space avaliable will limit their ability to include extras.
If you recall, DVD took a while for extras to catch on. I believe The Matrix was the first to really show what a DVD could offer (someone correct me if I'm mistaken). And that took a couple years after the release of the DVD format to make it to market.
Right now, Hollywoodland offers extras in HD. I'm not totally sure, but I believe it's the first HD disc to offer everything in HD including the extras.