I suspect that the extreme cost saving measures are to blame for the lack of a real interior redesign, just like the plant closures. I suspect there were more complaints about the old exterior than the interior, so they just thought they would make it look all new from the outside (to be fair there were bigger changes than just the looks on the exterior) and they'd be able to continue along selling trucks at the same rate with big profits.
They didn't account for just how staggeringly better the redesigned ram interior would be, and how much people buying these expensive trucks care about being in a nice place to drive for their $60k. If the ram interior was only a little better than the Silverado the brand loyalists wouldn't have jumped ship.
Also, in the interest of fairness. Ram did the same thing with their drivetrains and suspensions other than adding etorque and changing the air suspension a bit. Lots of carryover to save costs during redesign. The only difference is no other trucks came out with a new engine or suspension that really blows the Ram versions out of the water.
As has been said, ram did a great job of coming up with a truck that feels like a complete ground up redesign, and is a great overall package without any glaring deficiencies besides some QC issues.