Why should starwood try to go aggressively into a market that is fully covered by Accor? This would by something close to commercial suicide. They restrict themselves to Paris as the most touristy places with the highest number of visitors for the US. Any Euro outside Paris would be by far harder to harvest. The same is true for th UK, which is more a Hilton country. You find Starwood (with two exceptions) restricted to London.

Assuming it is true that starwood is mainly preferred by Americans then it makes more than sense to go to the places your customers go. And if you ask the normal American tourist going to Paris or going to the UK, 99% will confirm that the relevant capitals are on their itinery.