Since rogues are a dexterity based class usually, and shoving is strength based it doesn't really fit with the rogue to be a bonus action.
In DnD 5e it uses the attack action - so it costs a full action, except certain classes that are meant ot be making lots of attacks - fighter, barbarian, ranger, monk, paladins all gai nthe "Extra attack" feature at level 5, meaning they can attack AND shove as their action.
That's how DnD does it.. the classes that make more sense to be pushign people around can do it more easily than others. It makes sense. Changing it to be a bonus action doesn't make sense.
