Would you really want to use a... Polish TKS tank if you captured it? There's no reason to do so, there is a wealth of purchasable equipment that is superior.
Captured equipment was selected based on units the player would actually want to use. Remember they have a penalty of starting out as 0 experience, which also can make them hard to use. So we picked units that compliment the player nicely. There are no high armor units available in the early years of the war, so it's possible to get a Char B and Matilda II, both of which have incredible armor ratings for that period of the war. Eventually they fall into disuse, but for as long as they are around, they're pretty amazing. Same story with the Soviet tanks, eventually they will fall into disuse, especially once the late Panzer IVs, Panthers, and Tigers show up, but when you get your first KV-1B in early DLC 1941, it's the BEST THING EVER. lol
Seriously, 5 speed, great armor, decent firepower, plenty of fuel and ammunition capacity, what's not to luv?

Even so, it's like like you're upgrading these units to other Russian units. You can't turn your KV-1C into a KV-85 or a IS-2, you either retire it or if you like the experience and heroes on it, you can upgrade it into standard German equipment.