It does that every OS upgrade. I think the reasoning is that that particular developer toolchain was tied to that OS. Now that you're using a new OS, you shouldn't use the old toolchain. This particularly mattered in the 10.5 to 10.6 upgrade, because they extended gcc to support "blocks," which were tied into the Grand Central Dispatch implementation in the kernel.

It would be nice if they automatically detected, "Oh, you have XCode installed, so you would probably like the new version, too."