[donalm]

The main reason that SGML allowed closing tags to be optional was that it was 80s technology and storage was expensive.

To omit tags you needed a DTD that allowed the parser to infer where the close tags should go.

Enforcing the use of close tags in XML meant that building a parser was much simpler and that you no longer had to supply a DTD which I think were probably very instrumental in XML's success.