It's always been official that it was launching with 1.6, since the phone was announced. They have always said that updates to 2.X would come, but they couldn't say when. On the SE dev forums you see a lot of people ask this question too and it's really not much of a surprise. They developed the phone and their custom "ui" and media/timescape apps to run on 1.6. Which, might I add, is still the most highly used version of android on launching phones. It represents a more stable, and time tested build over 2.1 which just saw its first release with the Nexus One or 2.0 with the droid.
Take a look at mid-range Android phones that are releasing this year, whether from HTC or another manufacturer. Generally they are all running 1.6 and "future updates to 2.X" so it's really not SE doing anything different. Then keep in mind that every phone with out of the box 2.0 or 2.1 support is a Google phone, or a Google Experience phone and there is more reason for them to have those versions and others don't. The X10 isn't either of those. Look at the update roadmap for Android phones, I think you will find all google experience phones at the top of the list for updates..