There was a change in policies that occurred on Omni in 2012, which isn't necessarily bad, but nobody knew about the changes until over 2 years after being implemented. This resulted into gradual, unexplained changes in moderation and behavior from certain admins, which led to some conflicts between users who had joined without the changes in mind versus some admins. It piled up to the point where a third site was created, although it was finally decided that this new site would be more oriented into a programming group than a community news site.
It doesn't mean that Omni is ran worse than CW nor anything like that, because staff of each site is entitled to their views and are free to run their respective sites how they want. It's just that the policy changes left a gap in the community that needed to be filled in our opinions, so that's where CodeWalrus came in. All three sites being different is what makes the TI community more diverse. Besides, many people post on at least two different sites, often both Omni and CW.