The beast modes are alt modes - as I said before, the alt modes are typically secondary to the action Transformer (robot) mode. And again, it's difficult to incorporate articulation in a secondary mode without compromising the primary mode. For example, if you made any of the Animated Dinobots more articulated, then there would have to be a trade off because you're incorporating more moving parts. More parts = more expensive to manufacture = more expensive to purchase.
Remember that HasTak gives every Transformer toy a strictly defined budget. The toy cannot exceed this budget otherwise it becomes more expensive for HasTak to manufacture the product - an expense which would have to be passed onto the consumer. And this is something that HasTak is usually not willing to do because it's not good business practice. For example, you _could_ have a Deluxe Class Transformer with high articulation in robot and beast mode... but would most people pay $40-50 for it? Probably not. It's for this reason that often cooler features in toys are sacrificed in toys because they exceed the toy's budget restriction (e.g.
prototype Godbomber (compare with
actual Godbomber)).
Where articulation is concerned, priority is given to the robot mode over the alt mode, and that's fine by me considering the alternative.