Shog9 explains why it was closed in a comment:
Closing this as a duplicate, even though this is in relation to the old review system and the one I linked to is in relation to the new one, since for all intents and purposes the old system is now dead - none of these actions are tracked anymore outside of the new queues.
To respond to the more general question of why a old question could be closed in favor of a newer question, there are a lot of possible reasons:
Better formulated: A question that is written well and easy on the eyes is more likely to get good answers and more likely to withstand scrutiny, even if it's newer.
More views: Google matters. Pageviews matter. I will close questions as duplicates in favor of the question with more views because it increases the likelihood that question is used in hyperlinks throughout the internet (and there are other reasons, but this is just one)
Better answers: In this case, Thiefmaster's question had an answer from a Stack Exchange developer, making it canonical
More relevant: Sometimes, the way a question is asked can determine whether it will be superseded by another question. This happened in your case: The feature changed, and so your question lost some of its relevance that the other question retained.