Î don't pretend to have read or know the official reason, but in my view removing crits from spells "may" be with a view to addressing the balance in versatility.
As a Mage you have a whole host of options at your disposal, not just damage spells. Damage and conversely crit damage is in my view better served in direct weapon damage vs spell damage. Call it representing skill or whatever, but I personally see little issue in removing crits from spells.
In a computer game setting this is perhaps even more relevant where resting is much easier than in a PnP setting. In BG3 (before rations became a thing), you could do 1 encounter, sleep, get all spells back. Why be a fighter?
That said, I don't feel strongly about this change either way and most I know house-rule Crits anyway, so a change here might just mean group A rouserule crits instead of group B. *shrugs*.
