Because then his userbase goes from being an extremely small niche(chromium is probably less than 1%) to a much larger niche(chrome is more than 30%).
There are some real potential wins with being in Chrome, many of which are political, but there are no technical reasons for not being in Chrome to block Dart's adoption.
Sure, Dart's designed to have even more benefit when run in it's own VM, but that doesn't affect its reach.