Because BBVA by selling its 64% stake to MetLife eliminates the possibility of a bidding war. The market will need to sell to MetLife on MetLife's terms. Obviously if the BBVA wash't in such a financial mess they would not have sold, but they have and baring legal blockage the deal will go through with the price paid equal to what it was back in September when the bargaining began with BBVA.
The sell off today is due to traders who bought on the rumor and sold on the news. Considering the magnitude of the news, the volume is extremely low.
The big picture is much different. The market has not caught up with the new capitalization of $3 billion plus... yet. The market price will get closer to $140 as regulatory hurdles are cleared and the deal is finalized.
Actually with a couple of promised special dividends it works out to be very close to what the market has priced it at. I think 102 or something like that, but regardless of the figure it seems very low to me I don't understand why it was agreed to.