The main reason is that players paying $60+ for an AAA title do not feel eager to be forced to pay via MTX to gain significant important items or unlocks in the game. So EA responded by lowering the needed "grind" to unlock important characters by 75%.
See, that's where the problem lies. Users paid upfront and expect the complete game. They don't want to pay to unlock content (and let's not forget day 1 DLC's). The rule of F2P that content has to be free all the time was violated somewhat. That's one point, that they didn't implement the Loot Boxes right.
My second point is one I said often during my talks when Loot Boxes came up, like here:
If you add Loot Boxes as MTX you admit your normal monetization system failed - unless your core mechanic of the game is Loot Boxes. Like Hearthstone. Clash Royale. Or Puzzle & Dragons. You will notice that these games never came up during those discussions. Or why do Overwatch Lootboxes do not spawn that controversy? Think about it.
Basically, EA admitted their AAA retail flat fee model of $60 for an AAA game is declining, isn't making enough money to return a sizable profit so they add MTX.
Many games did this but EA of course brutally forced the MTX system on their game going over the top - while other games do it more wisely - as you should do too. Check out GTA V and their online Revenue, or even Assassins Creed IV Black Flag where I was part of the design team of the MTX system. They never had the backlash EA is feeling.
There are signs on the horizon that the old AAA model is dying. PC revenue is rising but most of that us on the F2P or MTX side, the AAA revenue is not growing compared to previous years. Even console $60 sales are stagnant to previous years or growth curves of previous generations.
So AAA games feverishly try to adapt f2p methods to raise their profits to cover the ever-increasing development costs - and most of these teams never worked in F2p. They need help. Call me if you need to educate your teams. You can add MTX to your AAA title which players actually love to engage in. Loot Boxes aren't the answer. Believe me. Been there, done that.