What immediately comes to mind when considering this question is that the the system is symmetrical - that is, it works exactly the same even if someone from planet 1 is transplanted to planet 2. It is not a subjective exclusion from the current planet. The energy of the current planet is objectively unusable. So what would make that happen? The simplest answer, for me, is that something in the transition from planet X to planet Y is necessary for it to become usable.
To find what that is, I would look at the physical layout of a solar system. What is between you and another planet that isn't between you and your own planet? This could be some object in its own orbit, or maybe the energy must be drawn through multiple planets to amplify. But my thought is that it must be drawn through the sun.
Why? Well, it's big, bright, and your planet's energy can never ever go through it on its way to you. The sun could act as a focus or an activator of another planet's energy, and that energy must be (inadvertently) drawn through the sun to be useful to a magician. Or perhaps the journey of the magical energy through the sun's radiation between planets activates it and it doesn't physically have to go through the sun, but the idea is the same. Something between the planets (the sun or some other arcane object) acts as a catalyst for magical reactions. 943
The energy around planets is the fuel, but it needs a spark somewhere out in space that just isn't available on the planet you stand on.
The other simple observation is the sheer distance involved. I liked it <PERSON>'s inverse square law idea, but I like it better the other way. The further the planet is from you, the greater distance the energy has to build momentum. If you draw it from a paltry 4000 miles from the center of your planet (or less if it's from the atmosphere), it is equivalent to a musket-ball rolling out of the barrel. But, drawing from a Pluto analog 5 billion miles away, your magical bolt comes screaming like an abused particle doing loops somewhere in a Swiss lab.
Think of it as potential energy in a gravitational field - except the gravity is the magician pulling on the magic. The further it has to fall (be pulled), the more time the force acts upon it, applying an acceleration the whole time. Even a small (whatever small is in magical terms) acceleration over 5 billion miles produces an inordinately larger velocity than an acceleration across 4000 miles. And translating "velocity" into magical power, that's the ballgame. The furthest source of magical energy on your planet just isn't far enough for any significant, noticeable power to be built up. 621