Docker is installed but Docker Compose is not ? why?

You also need to install Docker Compose. See the manual. Here are the commands you need to execute

sudo curl -L "https://github.com/docker/compose/releases/download/1.26.0/docker-compose-$(uname -s)-$(uname -m)"  -o /usr/local/bin/docker-compose
sudo mv /usr/local/bin/docker-compose /usr/bin/docker-compose
sudo chmod +x /usr/bin/docker-compose

I'm installing on a Raspberry Pi 3, with Raspbian 8. The curl method failed for me (got a line 1: Not: command not found error upon asking for docker-compose --version) and the solution of @sunapi386 seemed a little out-dated, so I tried this which worked:

First clean things up from previous efforts:

sudo rm /usr/local/bin/docker-compose
sudo pip uninstall docker-compose

Then follow this guidance re docker-compose on Rpi:

sudo apt-get -y install python-pip
sudo pip install docker-compose

For me (on 1 Nov 2017) this results in the following response to docker-compose --version:

docker-compose version 1.16.1, build 6d1ac219

If you installed docker by adding their official repository to your repository list, like:

$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
$ sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"

Just do:

$ sudo apt-get install docker-compose

In case on RHEL based distro / Fedora:

$ sudo dnf install docker-compose