Wizards and Sorcerers combined only account for about 15% of PCs and every other class gets armor proficiency* as part of their class. As such, the vast majority of PCs, including over 75% of the caster PCs have armor proficiency.
We are talking about the Magic Initiate feat here, it is a very small percentage of PCs that would benefit from Mage Armor through that feat, especially since both Wizards and Sorcerers (the only classes without armor) already have it on their class spell list.
As I said getting Mage Armor through the MI feat would be a small niche case for a Wizard or Sorcerer without Armor proficiency. The vast majority of characters would get more benefit from shield.
*note - Monks technically don't get armor proficiency either, but niether would they generally benefit from Mage Armor