This comes down to economics. 3/5 Aerialbots are using the same mold. 4/6 if you count Slingshot. Silverbolt is being remolded into at least Cyclonus, if not more later. Alpha Bravo will be remolded into Blades and probably Vortex later. All those extra uses mean more money can be spent on those molds because they will result in a certain amount of income for Hasbro over the course of a year or two.
The Constructicons, on the other hand, are six totally different molds, and those six molds are only good for this set. There will almost certainly be a G2 colored version of the set as an SDCC exclusive, likely produced at the same time as the mass retail green version, but the upshot is that these molds will be used for one thing and one thing only, and that puts the revenue potential at a certain point. As a result, simplifying the individual robots is what they did to keep the set within its established budget, focusing on making the combined mode as good as it could be. It's just one of the unfortunate realities of the toy industry: Tooling a steel mold is very, very expensive, and the more joints and hinges on a figure, the more separate pieces you need to produce, meaning more molds.
