Yes and no. There is no technical reason why Canon cannot put a better AF system in the 5DIII, such as the AF system from the 1DsIII, now that it has been supplanted by the 1D X.
But...answer me this - if there's no reason WHY the 5DIII shouldn't have a better AF system, WHY should the 5DII not have gotten a better AF system? There was the 1DsII AF, which had been supplanted by the 1DsIII. Heck, even the original 1Ds AF system was far better than the 5D's, and relatively old by then.
So...WHY not? Market-driven segmentation of the camera lines. Those reasons applied then, and they apply now.
Put another way, Canon is looking at the entire dSLR market as a whole, and seeking the way to extract the maximum amount of revenue from that market with the minimum investment - i.e. the most profit they can make. That's going to result in complaints from many people...but as long as the shareholders are not the ones complaining, Canon can turn a deaf ear to everyone else. You and me included.
I think also, when Canon launched the 5DII, it was easy, from a marketing point of view, for Canon to argue that the 5DII had the edge on the D700 in terms of resolution. Canon probably felt that it had enough of an edge having a 21MP sensor, as opposed to the D700's 12.1MP sensor, not to have to worry about developing a new AF system for the 5DII. (My understanding is that the AF system is exactly the same as in the 5D - I stand open to correction however.)
Given the state of play, in terms of sensors, it will probably be much harder for either Canon or Nikon to argue that one of them has a significant edge in terms of IQ of their sensors now. If Nikon does introduce a 36MP D800, and Canon a 22MP 5DIII, in the consumer market (i.e. non-Pro users) Canon will need to find a different way to differentiate the 5DIII. In the semi-pro and pro market, 80% of users may not even care about the difference between 22MP and 36MP - a niche will prefer 36 over 22.
That would be one argument why Canon might not be able to stick with an AF system that harks back to the 5D. Just how much Canon might think it is necessary to improve the AF in the 5DIII, if Canon believes it is necessary, is an entirely different story.
The 5DII is often bought as a kit (with the 24-105 f/4L IS USM) - because it serves as an "entry level" camera for those moving to either serious enthusiast or professional photography. That means it is often purchased by people who do not already have a big investment in EF lenses. For those purchasers, there is no inertia caused by having an existing investment in glass to protect - they buy mainly on the camera specs. That means a direct comparison to whatever Nikon has to offer is a significant factor. Both Canon and Nikon will be looking to get the "overall recipe" right to win those customers - since they have a high probability of investing significantly in lenses in the future. For that reason, I think Canon will need to reconsider the AF system in the 5D series, and do something which comes closer to competing with Nikon, but at the same time still leaving significant differentiation from the 1DX.