Well there are a few added levels of complexity, but the biggest is those two devices (the screen, the tablet) are not the same resolution.
Every VC game probably has two outputs: 480i and 480p and two screen sizes (wide/standard). The GamePad is neither of those. It is its own shape and resolution.
Also, keep in mind the reason each game has its own emulator: you can tweak the emulator for game-specific performance. That means EVERY SINGLE VC GAME MUST BE TESTED FOR EVERY REGION if you aren't going to run it in Wii Mode. Regression testing means more than a quick play of the game. If beating a game is like swimming a lap in a pool, a regression test is like swimming a lap in a huge lake.