As for new characters that were introduced in Rise of Skywalker, the two most prominent were both added to the script for the same reason: to shout “No homo”. Zorri doesn’t serve the plot, and is only there to be a romantic interest from Poe’s past. Jannah is meant to be a romantic interest to replace Rey (for the sake of Reylo shippers) and Rose. And there was no way we could allow Poe and Finn to have a romantic connection despite their chemistry, fans, and even actors’ desire… but it’s good to know there’s a line JJ won’t cross for fan service.
I still don’t see where people get Finn/Poe from. I literally don’t see any romantic tension between them whatsoever.
Besides, I feel like between Rey and Rose, we’ve already met the “no homo” quota for Finn. If anything, Jannah’s there to fulfill the stormtrooper rebellion idea, albeit extremely clumsily.
Then we have the new characters introduced in 2 and 3. Did most audiences really care about Holdo, Rose, the literally unnamed DJ, Jannah, or Zorri? Snoke had potential but that was cut off before anything could grow.
The difference is in TLJ the secondary characters move the character arcs along, whereas in TRoS they’re just kinda there.