I can see the frustration because an alarming number of people have said that there are no hype worthy Nintendo characters left to include so everyone defaults to their most wanted third-party character for the Fighter's Pass, and by default that excludes Dixie.
It really bugged me that a lot of K. Rool supporters defaulted to Banjo-Kazooie and not Dixie Kong when K. Rool was finally included, and how they were vindicated at E3 2019. You'd think a bunch of self proclaimed hardcore Donkey Kong Country fans would root for Dixie to get included now that K. Rool was in, but they went to the other big Rareware character instead. This was despite Banjo-Kazooie not being owned by Nintendo anymore—or, if we're being generous, because of that, out of a perception that they needed stronger support as a result...*sigh* okay I'll view it through the generous lens.
Now there are plenty of Banjo and K. Rool fans that have expressed their support for Dixie Kong, but it's far less rabid and passionate, it seems. Most of them popped in after E3 2019, said they supported Dixie Kong, and then left. I can't blame them, since Sakurai has politely asked to stop with the requests since the Fighter's Pass characters have been finalized and there's nothing anyone can do about it. This left Dixie Kong fans in a limbo—we can't be as loud and passionate as the K. Rool fans have been because it wouldn't look good due to Sakurai's statements, but we can't stop supporting Dixie Kong because our historical belief that Dixie Kong was an "obvious" and "inevitable" inclusion has been proven wrong in both Smash 4 and, sadly in Ultimate. However, it's nonetheless discouraging that more K. Rool and Banjo fans haven't joined us in at least regularly participating in discussion and hopes about Dixie Kong.
This limbo sucks, is what I'm saying, but within this limbo we could still be doing things like getting others to agree that Dixie Kong's absence from the roster is one of the most egregious, and not have that brushed aside. It's a shame that it gets brushed aside so much, though at least plenty of people outside of this thread do agree that Dixie Kong's absence is very noticeable.