Sakurai explains why Isabelle isn't an Echo Fighter in Smash Bros. Ultimate

When Isabelle was revealed for Smash Bros. Ultimate, many were surprised to see that she was her own fighter, and not an Echo fighter. If you were looking for an explanation on that, Masahiro Sakurai has provided the details in his latest Famitsu feature. The translation below is courtesy of Source Gaming.

Isabelle is based off of Villager. She pockets items and rides on a balloon swing. However, she is not an Echo fighter. This is because Isabelle and Villager have fundamentally different body types and personalities, so we couldn't have made her an Echo. All of her animations are different, not a single one is the same.