There were multiple reasons the A-43/T-43 never made it past the prototype stage.

First, the Soviets apparently had problems developing the proposed power plant for the tank, a V-5 diesel. The delay required to get the problems worked out came at a time where over 1200 T-34s were being built per month.

Second, it was essentially armed with the same gun as the T-34, the 76.2mm F-34. While effective, it was incapable of damaging the front armor of the new German Tiger, and only rarely able to penetrate the side. Because of this, the Red Army wanted to upgrade, which they soon did, to variants of the 85mm 52-K air defense gun. So this new tank was undergunned from the start.

Third, the armor, while an improvement over the already good armor of the T-34, was still unable to prevent destruction from the German 88mm. Meanwhile, the extra weight of armor resulted in the A-43 being somewhat less mobile than the T-34; arguably, this would make it a better target.

Finally, the time it would have taken to adjust factory production from the T-34 to the new A-43 was deemed unacceptable, particularly as it came at a time when quantity was proving to be just as valuable as quality.

All of these things combined to doom the A-43. As far as it being a "much better" tank goes, the T-34 is one of the legendary armored vehicles of WWII and arguably the best all-around medium tank ever. It's hard to see how the A-43 truly improved on it.