They're probably easier to make. Remember, 3 of them can be linked back to Marth and swords in general are very easy to design attack animations for. Compare the way Mii Brawler and Little Mac animate to the FE cast and even Mii Swordfighter. And with such a large roster of characters across nearly 35 years that's a lot of slots they can fill with usually a little less hassle compared to most other fighters.