Reasonable take...
FFxiv has the system it's built on and active information and saving is tight and a bit part of server load by necessity. The ARR engine was never designed to handle glamour and mounts playing a song other than the first mount song was unheard of. Everything needs to be built on an existing tech and rebuilding that tech is a careful process given how much it affects the ongoing data structure. A misstep can lead to major bugs down the road... Underflow and overflow risk increases the more you revise old systems.
Ffxivm however isn't built on ARR or 1.0 data structures so they can future proof by adding things out the gate safer.
It's not shocking they got it first... It's easier for them to do.
