The curves we're looking at are power required curves. The power required is the rate of doing work and in physics work is defined as "a force multiplied by distance/time"
So in our diagram the power required is described as drag x TAS.
We can say that the drag doesn't change with increasing altitude when climbing at a constant IAS but the TAS does increase, that's why the curve moves up and right.
Climbing at a constant Mach no. leads to a decrease in TAS but an increase in drag as alpha increases, overall this causes the power required curve to move up and right as well.