A lot of alternate methods of granting advantage have far greater costs/downsides for using them (and might only work for the attacker only, not the whole party + they're class specific while EVERYONE has shove) compared to a theoretical bonus action shove to prone, and shove being a bonus action is likely the exact reason why we don't have shove to prone implemented to begin with. It basically rounds back to one of the major arguments in regards to height advantage/disadvantage last year, in that its existence meant that the value of advantage-granting spells like Faerie Fire were diminished greatly for ranged characters. Sure, you say that shove being a bonus action is balanced out by the fact that it prevents you from using other bonus actions, but it's worth considering that
shove as a bonus action is so strong that it also simultaneously diminishes the value of using the other bonus actions to begin with.
There was someone on Reddit that brought up something interesting. It's shifted my thoughts towards the possibility that the reason why Larian is leaving bonus action shove untouched is because it was only ever meant to be a bonus action for the EA period only. This, for the sake of dispelling the idea that martials have less to do at low levels, and that the devs would want us to test shoving everything down every crevice possible.
There's an extremely high probability that assuming shove to prone gets implemented, bonus action shove may get changed to a main action upon full release. Otherwise, a level 5+ martial character with Extra Attack could theoretically use a bonus action shove to prone and then immediately get 2 attacks with advantage. There isn't really anything that can match that without some kind of steep cost, like Reckless Attack allowing all enemies to attack you with advantage afterwards.
