First it's very probable that they started working on them only after official release of macOS Mojave, as it's the known politic in NVidia for web drivers. And Mojave has been released only a month ago.
Then, they need to support Metal 2, and in software development you can't just implement and release. As it's a new API version, they'll have to implement it, write unit and integration tests for each new API element to support and execute those test, then fix all bugs detected...etc. Particularly knowing that they had quality issues on previous releases of Web drivers this year, they'll probably take a lot more care on it.
And they are also working to have CUDA support on Mojave. They probably won't release the web drivers until CUDA driver and SDK for Mojave is ready.
And as last thing, they are also probably working to deliver RTX 20xx support with the new drivers for Mojave, which will require its own amount of code and tests.
Clearly 1 month is a very short amount of time considering the work they have to do here. It'll probably take them several more months.