Maybe because they felt it was not worth it? I mean, the D800E may seem popular now but moire can be an annoying issue. Sure, people love sharper details and such, but there always is a tradeoff.
Depending on the D800E popularity, Canon may react in the future. I would say there is a good chance of that as well. The D800E seems to be doing well, but it is too early to say the long-term popularity of it.