I suspect that the ease of gaining an advantage is just a way to reduce rng. The current system of gaining an advantage such as attack from behind or high terrain is quite easy for the average player to understand.
Due to the fact that Larian targets a fairly wide range of players, RNG reduction makes sense, although you can argue whether it is the best solution (not too much in my opinion).
1) Adding a flat bonus is also a way to reduce the impact of RNG and it has less nasty side-effects than granting advantage.
2) The basics of dice rolling in D&D are dead simple. Roll a d20, add your bonus, see if you hit your target. If the game took three seconds to teach their players, it wouldn't be an issue. And if they do teach players what's going on, then a flat bonus is much easier to understand than advantage. If they really think their players are idiots and that they need to do the simplest possible thing, a flat bonus is better.
