They had a bigger army to equip and a more backward economy.
It would probably be better to describe their own equipment as being carefully tailored to their specific operating environment. We have no idea for example how a T-34 of that era (ignoring post-war models or upgrades) would have performed in desert or jungle, and the Soviets of that time wouldn't really have cared either since that isn't where they were.
When asking "what is a good tank?", you have to take into account what role it is expected to fulfil and what conditions it is expected to operate in. Being good in sandy desert and rocky hills is all very well but neither are high priorities when your main concerns are deep snow and boggy ground.
They needed equipment in their hands immediately far more than the ability to make it themselves at some point in the future. And Lend-Lease (and the Canadian equivalent called Mutual Aid)
was a sort of credit arrangement.
Tank Archives: Bovington: T-34 and KV-1 impressions
And:
And also:
Note in particular the statement "taking into account the manufacturing power of England and her dominions". The Soviets obviously considered Britain and the dominions (I suspect they were mainly referring to Canada in this instance) to have a very large and capable manufacturing industry.
This obviously went nowhere. I suspect the Soviet representatives were talking to fairly low level people who hadn't taken into account the practical problems that this would have run into.
To explain this point, consider the production history of the Valentine tank. Large numbers of this tank were produced in Canada (many of which ended up in the Soviet Union). However, this was a problem because a tank is far more than just some armour, a gun, and an engine. There are many off the shelf components which go into them, everything from nuts and bolts to wires and spark plugs. Many of the components for the Canadian made Valentines had to be brought from Britain as that was the only place they were available. This was a logistical problem under war time conditions.
This led to the plan for designing a new tank specifically to be suited for production in Canada, and Britain sent over a tank designer for this purpose. The new tank was to make maximum use of parts and components readily available already in Canada (which already had a large motor vehicle manufacturing industry) or could be imported from the US.
The result was the Ram tank. This resembled the Sherman, but was not a copy, but rather a parallel development based off of the Lee/Grant tank (as the Sherman also was). A US-Britain agreement to provide Sherman tanks to Britain (and the Commonwealth) however resulted in the Ram project being cancelled and the production capacity put to other use.
Producing T-34s or KVs in Britain would have required so much modification to the design to make it suitable for manufacturing in Britain - and remember we are talking about a complete manufacturing supply chain with multiple levels of suppliers - that there wouldn't have been much point to it. Britain had capable designers who could design a new tank from a clean sheet of paper probably faster than they could revise and adapt an existing design which had no commonality with British industry. All they needed was authority to actually do so, keeping in mind that an all new clean sheet tank would probably require some loss of production and availability of existing tanks while it was being phased in, the troops retrained, and the logistics pipeline refilled.
What would make more sense was to evaluate the T-34 and KV and take any interesting ideas from them and incorporate them into new British tanks. And that I suspect is what actually happened in the end.