To be fair, when the Scribd links were first implemented, 1) in-browser PDF was much worse than it is now; and 2) Scribd's interface was better than it is now. Over time, browsers got better at showing PDFs, and Scribd got more cluttered and weird.
I can't speak for others, but in my case, "difficulty of use" also refers to "you have to go through all these additional steps just to get to the document". I can't remember if that was only for grey-area, restricted-access postings, or if it applied to all Scribd documents, but I was conditioned to assume the latter by the time I saw them on HN.
Opening a PDF in browser is more convenient. That way the PDF behaves like another browser tab. Opening a PDF in an external viewer could be only one click and one alt+tab more, but it's less convenient and reduces usage. And it would leave one extra file in your downloads folder.