I’m tempted to say that the reason Casino Royale so outclasses every subsequent Daniel Craig Bond film is because of Eva Green.
After Casino Royale, Bond either couldn’t form a mature relationship with the women he slept with out of grief for Vesper, or the women in question were much younger than him or otherwise either shallowly written, and/or canon fodder for his motivation or to show the enemy’s threat.
Eva Green’s Vesper Lynd elevated the narrative by giving Bond a female counterpart and intellectual equal for a love interest.
Casino Royale had the powerful triad of an interesting hero, villain, and love interest. Vesper has never been properly replaced since Casino Royale, meaning all the subsequent Bond films have been missing a strong third party to enhance the drama of the story. Vesper Lynd was Bond’s equal in every way, they were perfect mirrors for one another from the moment they sized up one another’s measurements, and perfectly complimented each other’s strengths and weaknesses.
Bond films in particular are notorious for poorly written romantic relationships, but Vesper/Bond to me will always be one of the finest rare examples of relationship of equals that elevates the material.