Probably because Apple went Gold Master with 11.0 a few days ago and they're now working on future updates. Just because it went GM doesn't mean they'll actually release it.Well now this is interesting: Apple just seeded the first beta of Big Sur 11.0.1.
In fact, I suspect they will not release a GM to the public at all because the GM would expose anything related to any hardware they plan to announce at the rumored upcoming event on November 17th.
I could be wrong, but at this late stage of the game, I don't even see a reason to release the GM when the ship date of the new Mac(s) with Apple Silicon is approaching which will require Big Sur.
That theory goes out the window, of course, if Apple holds the event on November 17th to say you can order your MacBook with Apple Silicon for shipping on December 14th - In which case the release of Big Sur gets pushed back to some time shortly before December 14th.