How to upgrade Kali Linux

The apt-get upgrade command you have used will only upgrade packages that need no new packages as dependencies.

You can use apt-get dist-upgrade to include new packages in the set of candidates. Be aware though that using dist-upgrade will also delete packages that have been obsoleted by other, possibly newer, packages.

According to debian-administration some packages are being "kept back" beacause:

There we can see four packages haven't been upgraded, even though newer packages are available. This is because two new packages were introduced, and these upgrades depend upon it.

To fix this you must run :

apt-get dist-upgrade