"From the Soviet perspective, keeping everything Category A (T-90's, BTR-70's, with full combined-arms support) was way too expensive. You do that with offensive units acting as the Schwerpunkt."
That's only part of it. As the Russian defector who wrote under the pseudonym Victor Suvorov explained in his "Inside the Soviet Army", category B and C divisions were fleshed out with reactivated reservists and it made no sense to equip these units with hardware that the reservists didn't know. It was not only cheaper but also more practical to have them equipped with the weapon systems they trained with when they were conscripts than try to give them a crash course in how to use a more sophisticated but unfamilar weapon. The cat B and C reservists wouldn't be able to use new systems effectively so equipping them with modern kit would not only be expensive but inefficient.