Actually, there is a random function on computers. Given, its not TRUE randomness, but it gets the job done. To make it different everytime, they could make the date on the Wii the randomizer seed, so you have to start the game on the exact same second as last time in order to have the exact same sequence of numbers.
What they could have done was to program several types of approaches in there, and then shuffle between them. However, those approaches would a difficult sequence to program in [Think about a SHFFLED down air.The opponent might move, and every SHFFL down air would be exactly the same, so it would not be considered truely random]. It would have to choose when would be the perfect time for each tech, and there are so many techs. Also, you would HAVE to program a COMPLETELY different AI for every character in the game, which sounds like a VERY difficult job. I just want them to be done with the game now. If online goes in there who needs 1 player mode.
LarkOhiya summed it up perfectly. Also, even if they did advanced techs, when Brawl comes out, no one, not even the developers, would know all of the advanced techs, so you really can't upgrade the CPU.