If all pokemon got megas, a huge number of mega evolution would be redundant and objectively worse than others (like mega pinsir and megamence, for example), so it would feel like a lot of pokemon don't actually have a mega because nobody would use them.
So it would be a problem unless they managed to make each mega evolution unique and give a reason to be used to every single one.
And they already failed to do that with the 50~ they created
