Mostly because they already have an infiltration unit with the partisans. They don't need two different units that provide the same role even if they are a bit different. Especially when there is a lot more things they can do that is far more representative of the soviet army.
It's easy to say why not, but when there is money and time involved it's better to ask why.
Partisans are so weak in this game for me they do not exist.