I think a Transformer's body type plays a huge role in determining which alt mode they choose. Some Transformers are created with flight in mind, so they choose some form of aircraft that best suits their body type. Others are created with heavy layers of armor (think Ironhide) and they pick a vehicle that matches up with that. This makes the most sense to me. I mean, if all Transformers could choose their capabilities and alt modes without limitation, they'd all be multi-changers, combiners, or base/city formers. And honestly, what's the fun in that? In the case of Megatron, his body was essentially kitbashed from other parts in ROTF. And in DOTM, with his flight capabilities suffering from getting his shit handed to him by Optimus in Egypt, he had to base his alt mode on a land vehicle. Of course, this is Megatron we're talking about, and Megatron can pretty much do whatever the fuck he wants without having to explain himself.