The game with all the expansions has 3 ending conditions, all of which require you to keep an eye on the number of cards remaining.
Those endings are:
- All Provinces have been bought.
- Any 3 (2-4 players) or 4 (5-6 players) piles have been bought.
- (New in Prosperity) All Colonies have been bought.
It would discourage players from buying the last card to end the game if it's not a Colony or Province.
The idea is that you watch the piles as they go down and adjust your strategy to compensate. For example, if I think my score is good enough to win, it's to my advantage to buy out the last pile needed to end the game, even if it's something I don't want.
Requiring the game to keep going around the table after that would likely discourage me from buying said card; I would rather buy more higher-point cards, hoping that someone else will end the game.