One reason is that Lance Berkman (who is likely to return tomorrow) hasn't played yet. He's their best all around hitter and they've clearly missed him. Carlos Lee and Hunter Pence, who have played, have been abysmal, but they're too good to continue like this indefinitely.
Carlos Lee (who is making nearly $20 million this year) consistently hits .300 with around 30 HR and over 100 RBI, but he's currently hitting .104 with 0 HR and 5 RBI. Unless there's something physically wrong with him, he'll start hitting (having Berkman back could be the key). Within a few weeks, they could have Lance Berkman and Carlos Lee driving in the runs in the middle of the lineup and their entire offense might look a lot different.
The top of their starting rotation, with Roy Oswalt and Wandy Rodriguez, is solid and could be great. I thought that Rodriguez was a candidate for a breakout season and it's too early to say otherwise. Like most teams, the bottom of the rotation doesn't inspire much confidence, but it isn't terrible either with Brett Myers, Bud Norris and Felipe Paulino.
Aside from the Cardinals, the NL Central is weak. The Cardinals are 8-4, and have had great work from their entire starting rotation. Carpenter and Wainwright are both going to contend for the Cy Young Award again, but no one knows how Brad Penny, Kyle Lohse and Jaime Garcia are going to pitch for the rest of the season.
The Cardinals have underperformed in some recent years, such as winning only 78 games in 2007 after winning the World Series the year before. Matt Holliday and Albert Pujols also both missed time with injuries in Spring Training, and the Cardinals could be in trouble if either of them went down for an extended period.
The point is that the Astros (who have started winning) could look a much different team in a few weeks and may have righted themselves in time. If they start playing well at home (16 of their next 19 games are at home), Berkman gives them a jolt and Lee starts hitting, they could be around .500 by the time they go to St. Louis on May 11th. Yes, the Astros making the playoffs is still a long shot, but there are reasons to think they are much better than their early season struggles indicate.