It's not just because of piracy. SaaS has other advantages like making customer support easier, drive continued development with little risk, have easy cross-platform support, easier and more accurate usability testing, etc.
I agree with all of these. but, the main driver is piracy.
Look at products like 1Password. Everybody I knew was buying one license key and installing it on 5+ computers. Now that it's a service, you can't really do that anymore. The company definitely has more control over their software.
It's also one of the only ways to make money with open source, unless you want to make money on support, which is brutal for a small company with only a few employees.
