@IqbalArya: It's because of the English publishers. They sent MangaDex a C&D on select manga (which are all hot and popular at the moment) and MD is looking for a way to find a legal middle ground. Until then, MD is making sure to not make any waves and blocked all non-official scanlation content.
M+ has a partnership with MD and is one of the few sites that can upload official uploads but their policy is to only show the first 3 chapters and the latest 3. A few other official upload sites do this too.
So until a solution is found, this is what we gotta deal with...