The reason why this was done was solely because of mobile devices (phones, tablets, netbooks, etc).
Web traffic is increasingly coming from mobile devices. For example, mobile hits at my work's website is now nearly 40%.....and that's a 50+% increase from the same time last year.
This board's prime demographic (45-65 year old males) didn't grow up with technology--they've adopted it, and are increasingly going to mobile. Those of us in their 20s/30s grew up with technology, and use mobile a heck of a lot more than those who are older. Even younger people exclusively use mobile, and don't use desktop at all.
Anecdotally, I think most of us are sitting at a desk, reading/posting while at work on a desktop or large-screened laptop. But, even that is shifting with time. All websites were designed to be desktop-first, and worked/looked like crap on mobile. Now that the target demographic of this site is increasingly going to mobile, and younger users are almost exclusively going toward mobile.......we've seen a shift to a design that works better on mobile.
There are very, very few people who I haven't seen before posting on this board in the past 48 hours. In fact, I think the names I haven't recognized are ones bitching about the change. But, the change is here, it's staying/not going away.........just like very other site across the internet. You don't have to like it......but suck it up and deal with it.