I see there isn't a tag for some sports like darts, which probably wouldn't be as popular as football, NFL etc but there should really be tags if someone wants to ask a question related to that sport as it is quite a big sport in Britain.
Popularity shouldn't be a precursor in creating tags. If someone wants to ask a question on darts, or a lesser-known sport, a flag is raised, a meta post (like yours) is made, or the tag is created.
The reason there isn't a tag for "darts" is because no question has been asked about darts. See this for more information:
We do not and cannot proactively add tags to the site. Tags are only created when someone adds them to a question.