Apparently the reason behind the decision is simply that Omni wants to focus more on their other applications and these four were not worthy of as much attention. They don't rule out future updates, but it doesn't really look that likely.
For my own part, I can't help but think that OmniWeb's day has passed, but OmniDazzle and OmniDiskSweeper are still useful tools (not being a developer I don't have much to say about OmniObjectMeter). In any case, it's a nice gesture to the Mac community from one of its older members.
All four applications are free downloads from their respective pages: OmniWeb, OmniDazzle, OmniDiskSweeper, OmniObjectMeter.
[via Daring Fireball]