It's an issue with how compact the transmission and final drives are. In order to get better reverse performance, they need a new transmission and power pack, outside of increasing the vehicle width.

Starting with the T-64, the Soviets/Russians instead of one gearbox unit and two final drives (one per side) as used on T-55 or T-62, they are using a system that has dual planetary gearboxes and integrated final drives connected by a driveshaft which transmits power from the engine via the intermediate power transfer gearbox with no main clutch.

This system offers two gearboxes per side almost directly. The advantage of such configuration is that it’s simpler, lighter and more compact, saving space inside the tank, while being very reliable and durable.

Compared to the T-55, the side gearboxes only occupy approximately the same space as the epicyclic steering units in a T-55 and the gearbox connecting the two steering units in a T-55 are absent in a T-64, so the difference in the occupied volume is tremendous.

The disadvantage of this system is that it’s indeed compact. During the design phase, a design trade-off was made; only one reverse gear was put in place because the transmission and hull width could not accommodate a larger transmission. It was very tightly designed per original Army requirements, which dictated the maximum width of a tank. They simply could not add another gear without making the tank wider using this transmission and final drive setup.

The T-80 has a slightly different transmission setup to go along with the turbine engine (dual planetary gearboxes with dual final drives with five forward gears and one reverse); however this wasn't as successful due to reliability and fuel consumption issues, so the Soviets/Russians never really carried the design forward. Furthermore, the T-80 simply has way more power and torque available, which permits a higher reverse speed despite having one reverse gear.

https://www.reddit.com/r/TankPorn/comments/1i2xwhp/your_nonpolitical_opinion_about_the_t90m/