Think of PC performance as horses pulling a carriage. At 1080p, the GPU isn't too stressed, so the GPU 'horse' can run at full speed. Since the GPU horse is running at full speed, the CPU horse can also run at full speed.
At 1440p, that's almost twice as many pixels as 1080p. The carriage is twice as heavy, and the GPU horse can't go as fast anymore. As a result, no matter how strong the CPU horse is, it can only run at the speed set by the GPU horse.