Ike, Hector and Lyn are definitely more popular than Corrin and nothing bad happend
We can’t know what IS is thinking, especially because the devs love to break patterns after the community finds them, but I assume they’re both trying to give something to reaaaaaally old units (like Effie, Gaius, Serra etc) and to give new fancy outfits to popular units.
For example, I don’t think Brave Ike needed the resplendent because he already had a good art and was a beast gameplay-wise. But he’s popular so of course people will pay for him. But at the same time popular units like F!Corrin or Tharja still doesn’t have a resplendent, even when they need a new art and better stat lines.
Unless they choose units randomnly, and I doubt it, Corrin and many others will get their turn soon or later. They just prioritized other units for some reason