Because, the mount was what primarily was causing the boosting scenes expansion and driving negstivity into the game itself.
By simply not driving it to the highest difficulty drops the effectiveness alot as most average players can just get it themselves.
If we want boosting to die (and we do) we need to reduce the audience whos demanding it the most.
Collectors.
Its a cosmetic reward, we have other awards for pushing further that justify those good enough do.
Also.
Ksm is still under 25% of accounts that even have done m+.
This mount at best will be earnt by maybe 10% of the actual playerbase, so its where its ment to be realistically.
To make it higher ur just aiding the profits of the boosting communities.