I’m sole mod (not the original creator, but took over when they went awol) for the knitting community at !knitting, and I do my best to contribute a lot to the cross stitch & embroidery one at !lemmy_stitch too. This is coming from a history of running various niche online groups. So a few things I would advise:
First, just accept that some topics are too niche. They were too niche for Reddit as well, at one point. People got overexcited and wanted to mark their territory by setting up a ton of communities when they were new to Lemmy, but reality doesn’t work that way and a lot of those spaces just aren’t needed. We’d be better served combining posts from these into slightly more general combined communities, and perhaps leaving a sticky post in the tiny niche ones letting everyone know where to head to for that topic.
But if your topic is big enough to in theory get decent traction:
Be grateful for what users you do have. You said you sometimes get “few” replies, so make sure you’re getting to know those people and replying to them and continuing the conversation where appropriate. You don’t need a lot of users, you just need a few engaged ones to make for a nice community.
Recruit your friends. You’re a Chiefs fan, you probably know other Chiefs fans. Get them interested.
Drop your community link wherever its relevant. People don’t like having to put effort into finding new communities but if they just happen to come across mention of it, they’ll click. Obviously I’m not saying spam, but there are plenty of sports fans here and it’s bound to come up in conversation.
Crosspost. Any posts you make to a Chiefs community are probably also relevant to the wider NFL communities or maybe fantasy football players. And again this just gives more people the chance to stumble across the fact that you exist.
Ok these next couple are more involved, but they do work well!
Consider Mastodon. I have a craft-focused account there too, and if I have a question about knitting or cross stitch or whatever then the more answers I can get the better, right? So I use the fact that we can post from Mastodon, to a Lemmy community, combining the replies from both audiences in one thread. Example of what I mean here.
Create value. Could be by posting pillar content that’s actually useful (in your case could be some kind of statistical analysis, we all know the football nerds love it, but whatever will be long-term useful / interesting to your audience). Or it could be a regular community event or something ("predict the Chiefs wins/losses for the upcoming season and win something, etc etc).
Ask your existing users what they’d like to see from the community. Some things you try will hit and some will miss, but getting feedback is going to up your chances!
That’s everything off the top of my head and it’s already a wall of text so I’ll stop there. It is absolutely difficult to be a mod, it can be a lot of work to get to the point of just having an active community that doesn’t need your input to keep rolling. But if your community see you trying, I think that goes a long way. Hope some of this was helpful!