Given that the EU felt the concessions he won were substantial, you can see that the expectations of what the UK and the EU feel is fair and reasonable, are going to be quite different.
Really "substantial", would the concessions "he won" have ever been put in place, the EU just agreed in principle, mainly in the hope that the UK voters would be taken in and vote Remain.
Once that happened ( it did not though) then the "substantial" and "won" could become null and void.