This could be based on historical precedent, since the KVs were looked upon as increasingly inadequate by the Red Army as the war progressed. Sure, they had good armor and a decent gun in 1941 and maybe even 1942, but they were slow and prone to transmission problems. That they destroyed their share of bridges while crossing didn't help matters either. When the PzIV began getting the long 75mm gun, the KV was only a negligible improvement armor-wise over the T-34, while retaining all the aforementioned disadvantages.
On the other hand, the Valentine was one of the few lend lease vehicles that was specially requested by the Soviet Union. As a light tank, it was better than a T-70, and as the war progressed, its upgraded gun proved capable for its role. The Red Army was requesting, and receiving, Valentines into 1944.