I don't think XCX's 2015 release date helps its chances as DLC. Splatoon was released the same year and got straight into Ultimate's base roster; if X was popular it would've had the same fate. We don't know if the DLC roster was finalised in 2015 or (more likely) in 2016 and 2017, which would've given Sakurai time to gauge the game's reception/sales instead of making a decision based solely on its recency. Elma also doesn't fit any of the patterns DLC characters fall into. She's not new enough to be a promotional character like Corrin, not a missed veteran, not too late for the base roster and not a popular - or pushed - third party character. Bare in mind you could easily have 100% of the DLC be third party and have dozens of potential newcomers and stages left.
According to Source Gaming, the first proposal plan for Smash Ultimate was completed December 2015. So I doubt there was that much time to fairly gauge reception/sales for XCX.
https://www.sourcegaming.info/2018/07/02/smash_special_complete/
The focus of Smash Ultimate's roster was the "Everyone is here!" theme. Veterans. Sakurai himself said, "Don't expect too many newcomers." Inkling and Incineroar were the only two recent (having new or upcoming games) newcomers added to the base roster. So it's understandable why someone like Elma wasn't prioritized, with XCX not being an outlier hit like Splatoon or a mega-franchise like Pokemon and the fact that only so many unique newcomers were going to make the base roster to begin with.
But since X was among the few big games released in that 2015 time period when they were working on the proposal plan, it's reasonable Sakurai considered Elma. XC1 had just received a port on the 3DS and XCX had just released worldwide and had a year of DLC in Japan. XCX was relevant at the time, and Sakurai has chosen characters based on relevancy in the past.
We've seen XCX's content and it's comparable to something like Codename STEAM, not Rhythm Heaven Megamix or Yoshi's Woolly World etc.
What do you mean XCX is more comparable to something like Codename STEAM because I don't see the comparison, especially with how much time, effort, and resources put into XCX. It was a pretty big game, one of the few notable titles on the Wii U. Codename STEAM was a new IP on the 3DS. What makes XCX like Codename STEAM?
At this point I think rooting for Elma is less important than showing support for Xenoblade Chronicles X in general. We don't even know if the new Monolith Soft DIvision 1 project is XCX related, let alone a Xenoblade game (it could very well be a new branch of Xeno). If there isn't any evidence that people would be interested in another game like X there's no reason to try make one. I'm not trying to be pessimistic; I just don't want to spend another 3 seasons clamouring for something that might not even stand a chance. If there's a way to put this effort to a more constructive purpose I'd like to pursue it. I apologise for making so many negative posts recently.
I'm sure most every supporter here understands how you feel. We all want Elma in, and we all love XCX and have dealt with others ignoring XCX's existence or hating it.
And being real here, we all understand there's a chance Elma might not make it in. But there are many things in her favor, as several awesome supporters in this thread have already pointed out. So you don't need to apologize. You're just voicing frustrations most of us here are feeling, too.
This thread isn't just to support Elma being in, but to support other people that love Elma and XCX. And since we love Elma, we're going to keep supporting her regardless what happens, hence our optimism. We're fans, so we're going to root for what we want, especially since she does have a chance of getting in.