Ubuntu 20.04 LTS Likely To Ship With Linux 5.4 As Opposed To 5.5
The reason developers are targeting Linux 5.4 for Ubuntu 20.04 LTS is that the 5.4 kernel is the newest long-term support (LTS) release. With Linux 5.4 being LTS, it will be maintained by upstream longer -- until the end of 2021. If Canonical went with Linux 5.5, they would have to carry more of the burden in maintaining and back-porting patches for their kernel build.
Thus for Ubuntu 20.04 being LTS, it's logical they use 5.4 LTS. However, we are a bit saddened by the fact there are many great features with Linux 5.5 and Linux 5.6 is an outright major kernel feature release... Fortunately, it's easy for users to upgrade their kernel at their choosing.
As well, come Ubuntu 20.04.2 LTS in about one year's time from now they will end up pulling the kernel from Ubuntu 20.10 as their "hardware enablement" stack update. So at that point Ubuntu 20.04 long-term support users will have the option of what is likely Linux 5.7~5.8.
At least the Ubuntu kernel team is carrying a back-ported WireGuard for Ubuntu 20.04's kernel and we'll see what else might get back-ported for this LTS release.