To prevent having an infinitely-long "Tags" section in environments which have hundreds of tags, we only show the top few results in that section on the left. As the highlighted section states, please use the "Filter" bar at the top-center of the page to access all of your tags.
To add to this, the tags on the left should open out to show all the tags or there should be a way to select your favourite tags. I'd make an RFE @WaiKeat C.
You can create management zones and this will only show the tags of the relevant hosts, services, etc that apply to that management zone.