There are a couple of elements to this question.
Firstly, software which has just been released by the upstream project needs time to be packaged and built so there will always be some time before it becomes available in a packaged format. If a package has patches then more time will be needed to check whether the patches are still needed, and if so, re-written to apply against the new version. Compiling software from its source code can take a long time.
Secondly, most Manjaro packages are taken from Arch stable, and Arch packages also have to go through a process from being built, to entering their staging repo, then moved to testing, then finally to stable. Once it reaches Arch stable the package will enter Manjaro unstable, and so on through the Manjaro branches until it finally reaches Manjaro stable.
For more information about how the Manjaro repos work, read this post:
Some important packages will be rebuilt outside of this process. These are generally #security updates but still require time to build. For example, Chromium will take around six hours to compile (if it succeeds on the first attempt).