Jennifer Rubin explains that, even in the absence of a severability clause, judges try to save as much of a law as they can when portions of it are found to be unconstitutional, but in this case, the Obama Administration forced the court to void the entire Act:
Having determined that the individual mandate exceeds Congress' power under the Commerce Clause, and cannot be saved by application of the Necessary and Proper Clause, the next question is whether it is severable from the remainder of the Act. In considering this issue, I note that the defendants have acknowledged that the individual mandate and the Act's health insurance reforms, including the guaranteed issue and community rating, will rise or fall together as these reforms "cannot be severed from the [individual mandate]."
Oops. Not some crazy judge, but the administration was the source of the notion that the individual mandate can't be severed from the rest of the law.
But it's not just the administration; it seems Congress did its part to contribute to the invalidation of the whole statute. Judge Vinson observes that "the Act does not contain a 'severability clause,' which is commonly included in legislation to provide that if any part or provision is held invalid, then the rest of the statute will not be affected." He observes that this defect is not necessarily determinative. However, "The lack of a severability clause in this case is significant because one had been included in an earlier version of the Act, but it was removed in the bill that subsequently became law." Oh, now, there's a problem.
That is no small matter, the judge explains:
The absence of a severability clause is further significant because the individual mandate was controversial all during the progress of the legislation and Congress was undoubtedly well aware that legal challenges were coming. Indeed, as noted earlier, even before the Act became law, several states had passed statutes declaring the individual mandate unconstitutional and purporting to exempt their residents from it; and Congress' own attorneys in the [Congressional Research Service] had basically advised that the challenges might well have legal merit as it was 'unclear' if the individual mandate had 'solid constitutional foundation.'"