The OT may seem to have almost as much inversion as the prequels, but this doesn't account for the actual content of what is being said.
Most of Yoda's ESB dialogue when he gets serious is not backwards speech. Rather, it is stylized to sound Medieval, and also give it an antiquated feeling, but is grammatically acceptable. The dialogue sounds very similar to the way Tolkien writes and the way some of his characters speak, a sort of reverent pre-20th century type of flowery sentence structure.
"Told you, did I--reckless is he. Now, matters are worse....No, there is another."
There is nothing backwards about any of that. "Told you, did I" is a perfectly acceptable antique-English sort of phrase. "Reckless is he, like his father." Same thing. "Easily they flow, quick to join one in a fight." "That place is strong with the dark side of the Force--in you must go." To me, these don't sound "backwards" so much as they have stylized elements to them which inverts part of the grammer to sound pre-modern but is nonetheless grammatically acceptable.
The prequels follow suit a lot of the time, and contain some inversion-free sentences. However, key pieces of dialogue are not just inverted, but made backwards to the point where they don't even make sense--and this is what stands out, so it seems like the character hasn't been understood. "A prophecy that misread could have been," as someone pointed out--what? If this was the OT first of all that sentence would have been completely normal, but if it had to have an inverted element it would have been "Misread the prophecy could have been." And, it is the fact that there are unnecessarily-inverted sentences--even if only two or three per film--that further stand out, such as "to the forward command centre take me", which is grammatically fine as an inversion, but one senses unnecessarily so, and that in the OT this line would have been left normal.
So, firstly, even though the OT-PT contrast is not extreme in terms of ratio, the two or three unnecessary uses per film make a big difference, and secondly, I think most of the OT examples aren't true "backwards speak" but simply pre-modern stylization, and while the PT has mostly legit uses of this it has truely backwards nonesense sentences that the OT would have never had. I do feel like ROTJ has a lot of these problems too, but they are not nearly as bad. I guess once ESB was over Lucas (and maybe Kasdan) never fully grasped how his own dialogue was structured, or maybe it was Kershner who made it more sensible all along.