Why is chorizo spice-mix rarely sold in stores?
There are multiple brands of chorizo spice mix available commercially: Spice House, Savory Spice, sausage maker supply, etc.  Google a little.

Further, if you go to a grocery store with a good selection of Mexican food, you'll probably find some mixes there from Latin suppliers who don't have web sites.  Maybe not, though; all of those spices are standard staples for Mexican cooks, so they wouldn't see the need for a mix.

My recommendation, though, is that you make your own spice mix so that you can balance out the heat/garlic/sweet spices the way you like it.