It's mutual assured destruction. Killing the season will make fans that much unhappier; better to give themselves more time to negotiate and set the strike date after the season is over.
There is no strength in setting a strike date after the season is over. If baseball doesn't go on strike this season, the owners will lock them out before the start of next season. They will have all the leverage and the players union will get bent over the table and abused rather harshly. And the players and the owners don't care if the fans are "happy" about a strike/lockout. All they care is making sure they get something in the next deal that favours their side. If it means "mutual destruction", so be it. It's not rational, but it's the way it is.
Yeah, grum -- that's more or less my point. The lack of sanity applies to both sides of the dispute.