Why weren't containers popular before Docker?
It's a very broad question but I will try to answer you.
Docker was at first build on LXC, they switched to libcontainer later.
LXC is actually pretty hard to use compared to Docker, you don't have all the Docker related stuff like Dockerfile, Compose and all.
So I would say that container wasn't really a thing because of the difficulty of LXC.
As Wassim, I would say the main reason was that it needed motivated sysadmins, specific kernels (with OpenVZ and AUFS),...
Creating the same thing as a docker image was a complicated process.
Today it is a straightforward process, create a Dockerfile, just do
docker build -t mytag .
and you have created an image.
In 2004, you could not do that so easily.