Because many private server players have 8-100 days /played on the latest vanilla meta. This results in a very firm understanding of the game and the potential impact of various choices by the Classic team.
So for those who lack that substantial experience it’s easier to devalue it than to admit “I know less about this subject at the current time.”
Naturally, elitism and impatience can also come into play from those who have that substantial experience. In short, they’re mostly just protective of the game and feel the need to drown out the “please balance all the classes!” “please add LFG queues” posts as swiftly as possible.