HOW?!
second-in-command of the crew.
Timing - Yamato appeared in the middle of an arc few years ago, Loki appeared in the intro of an arc, after few years. We already have an whole arc between them .
Dream - Yamato dream was to become oden and travel. Loki's dream is to get the world - which could symbolically be tied to WG and Luffy's dream
Acceptance - Yamato was welcomed by Wano residents which became a reason she stayed to protect wano.
Loki is not welcomed by anyone in Elbaph. No one wants him.
Character portrayal - Yamato did get decent screen time for her character to fleshed out.
We hardly got some backstory on kid Loki and we are still a long way to get Loki totally fleshed out as Elbaph will go on for a quite a time.
Introduction and importance- Yamato was introduced for the first time in the middle of wano.
Loki was introduced or first brought up during WCI. And, Usopp dream ties with Giants.
They are two contextually different situations.
Not everything has to be seen with strict lense of power levels "only" because oda doesn't.
So, how Loki joining contradicts zoro as first mate?