JSON has a lot more optimization that XML never got. Which I think says more about general interest in XML more than anything. Even today my experience is that XML processing varies wildly from "perfectly reasonable" to "maybe I can just do this with regex instead" even with widely used parsers.
Also XML has a number of features to care about like attributes as well as elements, and also potentially about schema. It's also needlessly verbose. Even though elements open and close in a stack there isn't a universal "close" tag. That is, if `<Tag1><Tag2></Tag1></Tag2>` is always considered malformed, then why isn't the syntax simply `<Tag1><Tag2></></>`?
Also XML has a number of features to care about like attributes as well as elements, and also potentially about schema. It's also needlessly verbose. Even though elements open and close in a stack there isn't a universal "close" tag. That is, if `<Tag1><Tag2></Tag1></Tag2>` is always considered malformed, then why isn't the syntax simply `<Tag1><Tag2></></>`?