Lightroom’s Develop History panel is a log of all the work done on an image but it’s never written to xmp because:

  • To hide it from prying eyes. If you send files to a client, the edit history can disclose your clever methods or your time-wasting incompetence to the client
  • LR’s history data is of no use in any other existing application and xmp is as much about data interchange as it is about backup or moving work from A to B
  • There’s also a desire not to bloat the sidecars with what is merely a log.

Had Adobe included history, there would have been loud demands to make it optional, perhaps on an individual image basis, and probably lots of other inconsistencies too. It’s really not worth the development effort.