Earlier quoted context omitted.
With Sinatra you have absolutely no guidance as to what's good and what's not: there's no stack at all. I, too, am a bottom-up learner, but most people (in my professional experience) are not. Look at the success of Rails Girls, for example: beginners get waaay more excited and engaged by Getting Something Up There than they do learning the intricate details of HTTP.
>beginners get waaay more excited and engaged by Getting Something Up There Yes! That's why... require sinatra get '/hi' do "Hello World!" end ...is so amazing for newcomers. Even the file structure of rails is a bit too much for begineers. Everything in one file is so much simpler for trivial beginner "get excited" apps. Introducing someone to rails, they have to learn MVC, a templating system, an ORM, and more.
Rails is not a good starting point for beginner web developers who don't understand these software design patterns and aren't interested in learning them. If your goal is to just start making websites, like right now, then clearly something like Sinatra is a better choice.