Haki is widely misunderstood.
Haki does 2 things:
1. Makes things Hard - The harder something becomes, the better it is for attacking because harder things do more damage than things that are less harder. And it also acts as a defensive measure.
2. Other thing it does is it makes it possible to make DF users that AREN'T TANGIBLE or COMPLETELY SOLID interact able. This is what many don't understand.
Mr. 1 is Steel, therefore, Haki will not make it possible to hit him. If Mr. 1 were a Logia or had a body whose body wasn't solid (e.g. Katakuri), then it makes your body solid.
To hurt Mr.1, only strength is necessary. There are cases where Haki won't make it possible to hurt a DF user. Haki is not a fix-all to all problems. Someone like Buggy can't be hurt by Blades, therefore, even a Haki coated blade would be ineffecrive which is why Shanks said he can't die from blades.
So, how is he defeated? Blunt attacks and if Haki is introduced, it would just make thoes blunt attacks more powerful.
In other words, being that Haki is not always a solution to all problems. One has to utilise specific techniques or tactics in order to defeat some DF users. Like Haki won't be effective against Boa Hancock or Perona's ability or Sugar's ability.
There is also something significant about Zoro defeating Mr. 1. It's not Haki. It's a special technique called BOAT (Breath of all things).
For detailed information on BOAT:
To simplify, BOAT is a technique that allows one to convey their will into their blade (and don't mistaken will as meaning Haki, Haki and will are completely unrelated).