Hi Lewis,
Long story short, that version is no longer available in the Launcher, but if you desperately need it, you can contact Audiokinetic here.
The slightly longer explanation
We always recommend using the newest minor version since it's fully compatible with older minor versions.
For instance, you will still be able to generate SoundBanks with Wwise 2019.1.11, for a game using the Wwise 2019.1.4 Unity Integration.
* The reason we still show 2019.1.10 is that Wwise 2019.1.11 was released to update platform SDK and support for UE4.26 at the cost of not having UE4 support on macOS.
In light of this, we always stop showing every minor version release of Wwise 2019.1 as it's simply better to get the more recent minor release. The same thing happened to 2018.1 when we released Wwise 2019.2.
* The reason we still show 2019.1.4, is that the Wwise courses are currently based on that version.