Here's a theory:
Film and music are dominated by oligachies of a few firms roughly equal in power. Thus, it's pretty easy for them to pool resources to lobby, investigate and enforce their copyrights.
Software is dominated by a monopolistic oiligarchy, where one major player would burden the cost of a large-scale political/legal battle. And his taking on those costs would benefit all those smaller companies who put in nothing.
It might be more profitable for that one large company to invest the R&D and Customer Service costs that benefit only it, by requiring online authorization of its products and other security "features." This would mean that the smaller companies that couldn't invest that money assume the brunt of the costs of pirating, while the big player takes less of a hit.
This would likely lead to a consolidation proccess among the smaller players until they, too, could afford all the expenses related to the technical procceses needed to protect their copyrights. Or, that chaper technical solutions would eventually be developped by the smaller players.
O' course, I havn't slept yet today, so I could be way off. But isn't idle speculation about market structures fun?