[pionar]

(Disclaimer: I don't work for MS, so this may not be entirely accurate anymore.)

It's probably because IE is really just a UI wrapper around system libraries[0]. The changes for HTTP/2 would be made not in IExplorer.exe, but instead in WinInet.dll (and possibly URLMon.dll).

This is because IE isn't the only application that will use these new features.

EDIT: I should add that you don't just go changing system libraries in a patch Tuesday, you'd wait and throw them in a new version, hence the 10 preview.

[0] http://msdn.microsoft.com/en-us/library/aa741312.aspx