Why? Because vision after vision after vision has been seeing Daemon come to terms with his own self - his flaws, his pride, his manipulations, everything except his murder of his first wife anyway - and everything was perfectly set up for Daemon to realize that in fact he was changed and supported Rhaenyra for Rhaenyra's sake and his love for her and his brother's memory.
But no, apparently human emotional growth isn't good enough, there has to be a damned weirwood vision and a needless cameo of Brynden Rivers, the White Walkers, and even Daenerys herself before Helaena shows up (?) and apparently that is what he needed. So it's not about anything more than the (Old) Gods Will It and Daemon decides to accept his fate. Bah. If all that had happened after his reuniting with Rhaenyra and having the Rivermen publicly declare for her, that would be one thing but as it is, felt like the writers saying "Oh no, it's really all about Prophecy and not that he's had a change of heart."