It is a bit complicated but Wizards actually released a short article about the changes.
ORACLE CHANGES: AJANI'S PRIDE-MAY-TE (FUNCTIONAL)
To summarize it is a combination of a couple of reasons:
Due to changes to the way missing Beneficial Triggers are handled in tournament settings, a major reason for including the "May" doesn't exist anymore. Previously if there was a mandatory trigger both players would be responsible for remembering the trigger and if a mandatory trigger was missed it would require judge action to repair the gamestate.
Since the change, that is no longer the case. Nowadays if a "Beneficial" (gain life, creature gets bigger etc.) trigger is missed, too bad for the player that missed it.
Ajani's Pridemate is a popular card in the online Magic Arena game. It is also included in the "Tutorial" decks that are provided for free to players when they first get the game. This means that not only is the creature encountered fairly frequently Online, but it is also featured in most players first impression of the game (and for some non-paper players maybe even their first experience with magic).
Normally this wouldn't be that bad but the "May" trigger is quite obnoxious for pridemate in digital as each trigger requires player input, so something like swinging with a board of lifelinkers may require the player to click accept 20 times per combat. It can slow down the game quite heavily. Removing the "May" allows the triggers to auto-resolve with no player interaction (something that is not possible with "may" triggers currently).
It makes the card read slightly better in digital (and paper when it is re-printed).