After partner has bid two suits, responder may choose to play in the first suit bid despite the fact that he is longer in the second.
West bids 2♥ rather than passing 2♦ for several reasons:
1. If East has a good hand (one almost worth 3♦ over 1NT), game may be on.
2. If East has more hearts than diamonds, as is certainly possible, a 5-2 fit may play better than a 4-3 fit.
3. At matchpoints, if both 2♦ and 2♥ are making, the difference between 110 and 90 may be crucial.