In the various Transformers incarnations, we've seen it seems just about every possible Earth vehicle used as an alternate mode for a transformer. From supercarriers to motorcycles to tape decks. Why no road graders? A road grader is a pretty common construction vehicle. Plenty large enough to serve as a transformers alternate mode not to mention pretty high speed in vehicle mode with a large blade the robot mode transformer could use in combat. It seems a natural to me.
Why haven't they had a 1973 BMW? *shrug* They just haven't. To get an answer, I recommend you send a letter to every writer who has ever worked on the Transformers in all its incarnations. Maybe they'll get back to you.
Probably a matter of mass. When I look at pics online of Road Graders, they have a lot of open spots and holes, making them difficult to cram robot bits into and hide. Also, kids are very familiar with dump trucks and such, not so much road graders methinks. That said, never say never when it comes to Transformers. We've had Transformers bicycles and skateboards, so who knows?
This sums up everything you're thinking about, I'd say: http://www.youtube.com/watch?v=IHXJzA1Hn3o Mark
I saw the perfect vehicle to be a transformer alternate mode. It was a Caterpillar D-6 bulldozer with the elevated drive sprocket and the long, wedge shaped land clearing blade on the front. Plus a large two pronged plow on the back. That is the alternate mode I would use for Robots In Diguise character of "Wedge", the Build Team leader.
I've been thinking about this. On first glance it seems correct. But the Constructicons on the original transformers and the Build Team on Robots in Disguise both feature large tracked diggers. And both of them have large amounts of empty space.