Why does car lose acceleration uphill?

Automatic transmissions lose acceleration driving uphill because of the torque converter. The engine turns one side of the torque converter and the other side is splined into the input set of the planetary gears. The torque converter uses fluid to turn the input shaft in the transmission. When the transmission is locked into gear and going uphill the torque converter has to work against power losses from the tires on the road and when the engine is working harder the torque converter cuts or "shears" the fluid and it causes the transmission to overheat. Always make sure you have enough fluid in the transmission