We know Lockdown was sent by the "creators" to bring back Optimus Prime to "restore the cosmic balance." Lockdown mentions how "Autobots, and Decepticons, like little children..ALWAYS fighting." Optimus and Megatron were their factions respective leaders in X million years at war with one another -- they seemed destined to be at odds forever (Megatron living on through Galvatron). Since the two have such an extensive history that spanned years and years, and both ending up on earth in the live action movies, would the creators have just as much invested in retrieving Megatron back as they did in Optimus? One would think so, but perhaps with Optimus being a Prime, he is more worthwhile or valued by the creators..