I don't think that hbs are becoming obsolete but are they maybe pbs are cheaper to print?
I think the answer is partly this - those published in hardback (particularly a debut) marks the sign of confidence from the publisher and booksellers, this book is going to do well. It's like a signal to the publishing world/readers - hey, this book is going places. We all know that's not always the case. Some books come out in hardback and do not sell great. On the other hand, books published in trade paperback can zoom up the charts and make all sorts of lists - like THE EDUCATION OF DIXIE DUPREE.
And interestingly, the variance in printing costs aren't all that much different from what I've read. In actuality, the printing cost between hardback and paperback is only a few dollars, but it's the rest of what comes with it that makes them more expensive. (editing, marketing, advance to the author, etc.)