Because it's always one of the later forms to be finalized by Intuit.  They are busy with 1040 duties.  Can't be everywhere at once, you know.  A similar question is why haven't my 1120-S clients just printed out a year-end report for me, since that's all the data needed for doing the return (at least to get the draft started).  The answer is the same -- because they are busy with other stuff.