I would agree Ram should have gotten some SRT love a few years ago. Really though, the Durango came "for free" development wise since it is on the same chassis as the Jeep GC. And adding a new engine onto a platform takes some time. 4th gen ram was nearing the end of its run, so effort probably not worth it. Now that we have the 5th gen, I'm hoping they planned for at least the truck duty 6.4 but if Ram gets a new I6 turbo, TR 426, and TRX 6.2 supercharged there's not too much to complain about.
Decision not to put anything over 5.7 in the 4th gen could have had something to do with manufacturing/supplier capacity as well. They had a lot of SRT 6.4s to supply and initially supercharger supplier couldn't keep up with hellcat demand. Also not sure duty cycle differences. Yes durango SRT can tow well into the 8000s but may not be satisfactory for Ram truck engineers. HD 6.4 has lower rating and differing components for a reason even though it uses same block i believe as the 6.2 supercharged.