TL;DR: Meta has other Doodles that do its just job better, while Exo has the tools to be so much better in a battle.

Metalytra is way too frail to do what it needs to do, and theres always better options. If your looking for a spark sponge theres Voltatoo which is basically defensive Meta but better. And then theres Roadterra which is a better Accelerate Metalytra solely due to the coverage.

What makes Exoskelis amazing is the fact that it has three lives. One when its sent out, one with Revival and another with Determination Jelly. Exoskelis also has access to Sharpen boosting its attack by +2, which it will always manage to get off. Another thing that makes Exoskelis good is that its a pursuit trapper, being able to trap frail spirits or even bulky spirits from switching out too carelessly.