From the NY Times:
The reason is simple. Wine is a cash-flow business, and all along the pipeline, from farm to production to sales, cash is not flowing. Growers are behind on sales of grapes, which are fetching much lower prices than last year. Sales are sluggish for wines retailing at $15 a bottle and higher. Meanwhile, distributors, restaurants and retail shops are reluctant to buy more wine, preferring to sell through what they already have.
The whole thing is worth a read.