Great points, your article was spot on. No, definitely that's not the main issue, although it is one. You can't keep reinforcing why the user is signing up throughout the process the same way you can on the web. All mobile sign-up screens pretty much look the same and I see a lot of users look at it and just delete the app, or delay until later at which point they forget why they downloaded it. And even when they get past it, if they see something else they perceive as a "this is going to take a while" moment, they are going to leave and delete your app. On the web, the time to signup and get use from your app is faster and perceived as faster so I think given a choice between the two, more users would do it on the web.