Because healing word is a bonus action that you can cast at a 60' range, and it doesn't just stabilize someone, it pops them right back into combat. It's not comparable.
I think OP's suggestion is a good start but doesn't go far enough. Healing word fundamentally changes encounter design in 5e because, as OP correctly points out, it makes the "whack-a-mole" strategy of only healing people when they drop to 0 HP the most efficient, effective way to heal, and drastically lowers the stakes in combat because rescue is usually just a bonus action away. And the caster still gets to take another action. It's almost a get-out-of-jail-free card.
If you run a combat where the party has, say, a Bard and Cleric both with healing word, it is extremely difficult for the party to be in peril. From a storytelling perspective, I think healing word lowers the stakes too much. I would like to see it nerfed in OneD&D, but instead they are doubling down by giving Bard's a new ability where they can effectively do healing word (more or less) as a reaction. So then a bard will be able to pick two members up and still take an action (and if they have the medicine skill, that action could be to pick up a third member).
Edit: something as consequential to the story as saving your bleeding out teammate should at least require your action, IMO. Not an afterthought.