I don't know how Debian maintainers decide which version is included in a particular stable release.
According to this:
https://wiki.debian.org/KDE#KDE.27s_software_in_Debian
Debian stretch had plasma 5.8.6, and buster has 5.14. 5.12 is in between the 2 Debian releases.
Debian Unstable currently has plasma 5.17, even though 5.18 LTS has been out for some time. Who knows if Debian Bullseye (now Testing) will get 5.18 eventually.
Depending on the timing of various releases of Qt versus Debian, it might be easier or harder to backport Plasma LTS to MX's current release. Plasma 5.18 was not possible for MX19/Buster.
Not sure if 5.12 could be built on Buster base but I agree with you that going with 5.12 LTS might have led to even more complaints from various MX users, since it would probably have even less finish and fewer features.
For instance, the blur desktop effect only came out in 5.13.
https://www.techrepublic.com/article/ho ... -kde-5-13/
If a user is into Plasma for eye candy, going with 5.12 means you miss out on all the pretty Kvantum themes utilising blur (and blur effect in konsole).
Maybe it's a superficial reason, but it's part of the fun of Plasma, IMO.