I'm assuming it was in combat, although typically I allow actions outside of combat. I see on StackExchange a discussion of Readying spells that can be cast with a bonus action that is worth considering. Imagine a player does this -
- I use my action to Ready holy weapon, trigger is I say "go"
- Say "go", casting holy weapon without using my bonus action!
- Use my bonus action to dismiss the spell, and cause the weapon to emit a burst of radiance, dealing 4d8 radiant and blinding creatures of my choice within 30' that fail their Con saves
That might not be the most effective use, but there are many such cases. You can see that allowing a player to use their
action to Ready a spell that can be cast with a bonus action, allows them to use their bonus action for something else, so that in that turn they might get two bonus actions. I think the designers want to be able to rely on bonus actions being exclusive. Being able to rely on them being exclusive, gives them some freedom (they don't have to balance for possible multi-bonus-action turns.)