Probably two things. On one hand, our theoretical understanding of quantum effects may not have come far enough to write a traditional simulation program. Or do

Probably two things. On one hand, our theoretical understanding of quantum effects may not have come far enough to write a traditional simulation program. Or doing so would take years or even millions of years, or both. Quantum computers can simulate things that are not possible with traditional computers. These problems are in the complexity class BQP. I want to point out though, that this is very likely a result of our limited understanding, and not something that is theoretically impossible.