I think is part of it. It's hard to pirate something you can't get a copy of.

Other things is that the web is more universal. Say I have a great client app. It works perfectly on a client Windows machine. If I want to tap the larger market I have to have an OS X version, an Android version, iOS (depending if I didn't target both when doing the OS X version), and a Linux version. To keep all of these people happy, the versions should be pretty much the same. That's hard to do with client software on multiple targets.

The web targets the lowest common denominator: browsers. Fortunately as those go, the VM that is the browser is pretty powerful. Sure I might not be in my preferred language, but I can get things done. If done right, powers are added as the device becomes more capable my leveraging existing frameworks like Bootstrap, etc.

I think there are two other factors as why web apps are dominating client apps. The first is distribution cost. Even with AppStore and Android market, it's still very hard to get your app to the users. People browse to hundreds of different websites everyday, but very few try out tens of apps in their phone. The second factor is that the web is intrinsically a lot more connected. One web page/app can easily link to another with, doh, a link. The linking/back button semantics are well understood by all the users.

"not suffer from piracy" really means "has more power to lock in and control people", and yes, that's a major factor.