Chrom has an outfit, yes, but take a moment to consider the first time he ever had the outfit. Back in Smash for Wii U/3DS, he was present in Robin's Final Smash, then got an outfit in the DLC line-up. It took a sequel for him to be playable, which is where he is now. Compare him to Rex, whose Mii Outfit debuted before any playable appearance, and the same can be applied for many non-playable vs. playable characters with Mii Outfits. It all correlates.
The circumstances behind Rex's Mii Outfit is a bit different from the norm though. Sakurai gave a reason as to why Rex couldn't make it into the roster, be it base or DLC, so the outfit is a consolation prize. However, it's possible that Sakurai may revisit Rex's concept and include him should there be more DLC beyond the Fighter's Pass, if only because the dust will settle by then. It's either he commits to giving Rex the Isaac treatment, or throws another bone to the Xenoblade 2 fanbase.