Why does mxlinux fall behind Debian . . .
MX Linux is a Community based flavour of Linux. It is done in co-operation with another branch of Debian called antiX.
When an upgrade to Debian is released, the developers at MX Linux have to merge their system with that new version.
They have to check to see that all the packages, modifications, and tweaks will work properly with the new kernel.
Because they are volunteers, every thing must be done on their own time. They don't get paid, and they still have to buy their own computers, internet access, &c.
If they do find a compatibility issue, it will be even longer, before they can release a new stable version.
If you are using MX Linux, and want to help, why not join the group and offer to volunteer?
https://mxlinux.org/about-us/
Cheers!
Naught