Array literals in programming languages are pretty much as compact as you can get (start, end, one character element separator), and the type system or a little code often allows you to fix a 1:1 mistake after the fact. This attribute is now a number or an array of numbers.
In XML if you want a properly formatted list you have to use child elements, which gets expensive really quickly so people balk. Especially if you already failed the attribute/child test by mistaking the value for 1:1 when it turned out to be 1:many.
We also already have HTML class, style, and on* attributes as prior art that we use to convince ourselves that one more will be okay. Basically XML gets it from both ends. XML makes entity relationships and irreversible decision, in an era where UML existed and harped constantly on it.
I don't know if there was any fixing it at that point. What I do know is that a lot of us shot meaningful glances at each other and tried as hard as we could to find something else to do until the dust settled. But you really could not avoid XML in the 00's.
This is an almost universal disease of people trying to write XML grammars (or database schemas before that). Everyone thinks they can unambiguously decode multiple pieces of data stuffed into the same fixed-width field, and pretty much all of those people are wrong. Over, and over again.
That they never learn is a testament to the developer's ability to make fixing their Kruger-Dunning moments Someone Else's Problem. Get promoted or switch teams before the consequences are unavoidable. You can make an argument that even Tim Bray did this.
There are a lot of reasons XML was never going to succeed, and this is far from the most important, but it's a substantial contributing factor.