As with most things, it probably comes down to personality as much as Engineering.

If the impatient tribe wins, shouting 'YAGNI!', they may then have to shoehorn in features into existing syntax. For example, html YAGNIed away PIs and external general entities, leaving a syntax gap for server-side includes and so on.  People say YAGNI when they sonetimes mean IAGNI.

If the PTSD/OCD catastrophist tribe calls the shots, there may be so much emphasis of edge cases or 'completeness' that any simple core gets completely obscured. To wit XSD.

Or if the pragmatic/pioneer types do it, then the quest for the middle ground or sweet spot may overide any opportunities to radically  consolidate or to modularize for future use. Perhaps JSON.