They are basically trying to clarify the language. Saying “every citizen” is very vague meaning it could just be someone who lives in the U.S. So what they are trying to do is make it to where only U.S citizens mean legal citizens are allowed to vote if they are 18 and up. That is why they are changing it from “every citizen” to “only citizens”.
I hope this helps, I was a little confused ngl:)))