I always cringe when this topic comes up because I have consistently said that I am not theologically gifted and educated enough to discuss monophystism. In Patristics 202, we zoom through it in 3 weeks as part of a discussion of Chalcedon before and after. There are just so many more worthwhile things to cover such as St Athanasios, St John of Damascus, etc. But I will try to offer a few comments.
Monothelitism, if I recall correctly, was a political heresy invented by Patriarch Sergius (not the one everyone on this site hates!
). It was framed in a Neo-Chalcedonian (ie. those who accepted Chalcedon after the fifth council condemned the Three Chapters, cf. Leontius of Byzantium's theology) millieu as an attempt to speak of one will in Christ as in one principle of action, so that Christ would be united.
St Maximus the Confessor noted that a nature, in order to be a real thing, would have to have a will or an energy, because a real nature has to operate. So he found Sergius's reasoning to be lacking and shortsighted, and hence declared that no, Christ has two wills and two energies.
Non-Chalcedonians, on the other hand, seeing the nature of Jesus Christ as being one incarnate (composite) nature after the union coming from the two diverse natures (human and divine) saw union of God and man in a different way (as a composite nature) and hence the idea of energy or will is foreign to their whole theological system. In other words, they are not concerned with whether a nature has to have an energy proper to it to be a true nature, nor are they interested in wills or energies in Sergius's way of thinking because Sergius was creating this idea of operation in order to speak of a unified Christ. Non-Chalcdonians, believing the Word taking flesh effected the one Christ, would need no exposition of one energy to maintain one subject.
In other words, you are taking the question out of its historical development and context and hence coming to a novel conclusion. Someone once said (I will have to ask Fr Behr), "conclusions without the arguments that preceed them are ambiguous at best." Of course I mean this in a spirit of fraternal charity.
anastasios