The Eagles, Penguins, Yankees, and Dodgers were all established in their cities in 1967 or earlier. They are all deeply ingrained in their city's culture/identity with fandom passed down from generation to generation. My friend is an Eagles fan from Pennsylvania. So is his dad. So was his dad. And my friend's 6 year old daughter has a bunch of Eagles stuff and is becoming an Eagles fan. That fandom runs a hell of a lot deeper than Rams fandom in St. Louis.
The Rams were here for 20 years and were the city's 2nd chance at an NFL team. I have memories of watching the Joe Montana Chiefs when they were the closest thing to a 'local' NFL team before the Rams got here. And then the Rams were back to LA before I turned 30. I also have fond memories of the Greatest Show on Turf during my formative years, but none of my childhood friends had parents who grew up as Rams fans and none of my high school and college friends had kids who were old enough to have become Rams fans before they left. The Chiefs were in Missouri (and on over-the-air TV in St. Louis) before and after the 20 year St Louis Rams experience. It's really not comparable to teams who have been the hometown team for 50+ years.
Additionally, the Chiefs and Rams never had a rivalry. They played 6 total times when the Rams were here and were in opposite Conferences. There were plenty of people who rooted for the Chiefs as their AFC team while rooting for the Rams as their primary team.
I'm far from a Chiefs fan. I'm a Broncos fan who is utterly and resoundingly tired of that team. But I completely understand why so many people in St. Louis are Chiefs fans.
St. Louis and KC certainly are rivals , I was in college in Rolla during the World Series and KC fans were, and still are, as obnoxious as it gets, and it wasn't just Cards/Royals, they despised the football Cardinals as well - and I certainly reciprocate
I had three kids play and there was palpable dislike between club soccer teams from the east and west side of the state
I was Rams season ticket holder for 25 years, I know plenty of people who are still Rams fans
my best friend is still a Rams fan and goes to 2 or 3 games a year, I hate the Rams now, but I understand those that are still fans (it won't stop me from razzing them, though)
just because a team is in the same state doesn't mean "oh, root for them now"
pro football has been in St. Louis for 53 of the 65 years that pro football has been in the state
we had our team, they had theirs, but hey, now let us throw that out a window and all sing kumbaya and root together? f*ck that
BTW, the Bronco's are my 2nd team after the Bengals, my daughter is a huge Broncos fan, we went to Cincinnati to see them play each other, it was awesome