Regarding the supposed barrier that is installing the app: I don't know. For a paid service (as we are comparing apples to apples) the signup process on a web app can be more tedious than clicking a button, entering your mobile store password, waiting couple of seconds, and tap "Open app".

My candidate causes of the current situation:

- People are used to paying very little for apps. On the one hand, apps have been commoditized, yes, but on the other hand, web apps can be a lot more useful and productive than a phone app. We'll see what the iPad Pro and Android successors do about that. I'd suspect that there will serious money be made in enterprise apps.

- Competition (just swipe left and you find other purchase options) and reviews (highest ratings always wins) - I think OP got that right. You CAN do research about web apps before you subscribe to one, but that information is never complete and you can easily shrug off a bad review on some forum and buy that subscription nonetheless.

- Lack of trial versions. I would never buy a $10 app that has one 2-star rating. I would try it out, though. Note that almost every web app has a trial mode - it's a huge conversion driver! That one's completely missing for apps. I think that's also the reason why freemium has become predominant.