As you may know, Ubuntu Touch is a relatively young system, being officially supported on three devices only: Bq Aquaris E4.5, Meizu MX4 and Nexus 4.
And like other new operating systems, it lacks some important applications like WhatsApp, that are already available for Android and iOS.
But Canonical plans to implement WhatsApp and Dropbox as operating system services, like ssh, systemd, cron etc, to reduce the porting costs in bringing these services to the Ubuntu Touch OS.
Also, since it the Ubuntu One has been discontinued, they need to use third party cloud services like Dropbox.
We don’t know when WhatsApp and Dropbox will become available to Ubuntu Touch, but the two are most likely among the important apps not yet available for Canonical’s mobile OS.