A number of reasons.
Starting with the lower sales of Multichannel downloads - typically 10% of what a Stereo edition of an album would sell.
Add in licensing of albums separately for Stereo and Multichannel downloads by the major record labels, added storage costs (a 5.1 channel album takes up 3x the space of a Stereo edition), extra time in preparing the Multichannel download (extra channels to transfer, master and quality control, etc.).
The good news is that there are music download sites offering Surround Sound music downloads. In a few cases that even includes special combination pricing, pioneered by NativeDSD Music, where you can get both the Stereo and Surround Sound editions of an album for just $2-$3 more than the Stereo or Multichannel edition by itself.
But Brian....isn't it a self-fulfilling prophecy....there are fewer multichannel downloads than stereo downloads...and if you look at rock multichannel downloads.. the number gets even smaller..so naturally there is going to be a smaller number