Second, if you look at the specs for a transistor, you'll notice that many of the ratings are specified as CURRENTS. Although we often use voltages as a reference in digital circuits, deep down inside they're really ANALOG circuits, which require a certain amount of current to operate. If you design for a higher operating voltage, you still need to run a minimum amount of current through the transistors to get them to switch.
Do the math:
3.3 V x 0.010 A = 0.033 W
24 V x 0.010 A = 0.24 W
Multiply those Watts times thousands of transistors in an IC and you can see where the power requirements (and heat) could quickly get out of hand.
Nicely! laid out