apt - Why is gnome-todo not installed although it's a dependency of ubuntu-desktop?

Why is gnome-todo not installed when it is a dependency of ubuntu-desktop meta package in 18.04?

$ apt-cache rdepends  gnome-todo
gnome-todo
Reverse Depends:
  libecal-1.2-19
  vanilla-gnome-desktop
  gnome
  ubuntu-desktop
  libgnome-todo-dev
  libgnome-todo-dev

  libgnome-todo
  libgnome-todo
  gir1.2-gtd-1.0
  gnome-todo-common
  gnome-todo-common
  gir1.2-gtd-1.0
$ apt-cache policy gnome-todo
gnome-todo:
  Installed: (none)
  Candidate: 3.28.1-1

  Version table:
     3.28.1-1 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
$ dpkg -l | grep ubuntu-desktop
ii  ubuntu-desktop                             1.417                                                    amd64        The Ubuntu desktop system
$ dpkg -l | grep todo
$ sudo apt install ubuntu-desktop
Reading package lists... Done
Building dependency tree
Reading state information... Done

ubuntu-desktop is already the newest version (1.417).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Update: This situation probably results from an interrupted and subsequently repaired upgrade to 18.04. Would doug's suggestion (sudo apt remove ubuntu-desktop && sudo apt install ubuntu-desktop) restore all the default/recommended packages that are installed during an upgrade?

gnome-todo is a recommendation of ubuntu-desktop. As such it can be removed without removing the ubuntu-desktop meta-package.
Also to note that it will be included on the 18.04 release image.

In this case it should suffice to remove & then reinstall the ubuntu-desktop package

sudo apt remove ubuntu-desktop && sudo apt install ubuntu-desktop