The possible reason to why Valve didnt add this great looking weapon is possibly because
A. This dosent have the Gold Certified star.
If you go around and look at the items on the workshop you might notice that most items have a Gold Star at the side but unfortunately here this Item dosent have it
Or B. Valve wasn't/amazed by this item
This could be the most easiest one that people think since there was so much good items but they only picked a handful of items in the update

Don't get your pants in a twist by going "NO $HIT FATA$$ CU#T" because it seems like most people are confused on why this wasn't added to the game unless anyone has any ideas why it also could of not be added that would be nice to know