Remember that an engine produces less horsepower (normally aspirated) as altitude is gained. In conjunction with the engine producing less horsepower, the airfoils become less effective. The definition of Vx is the speed or AOA that will produce the greatest gain in altitude within the shortest distance. Now, as we gain altitude and the effects of less dense air come into play we are going to have to hold a higher pitch attitude to achieve Vx or the speed or AOA that will produce the greatest gain in altitude in the shortest amount of time, due to a less effective airplane. Essentially, our reserve thrust is depleted. Vx and Vy change are consistently changing as you climb. Vx increases and Vy decreases until they eventually meet at the absolute service ceiling.
Hope that helped
.