Simple. The character arc is missing. There's no process of Alex coming out to himself, no grappling with whether to go straight into politics or take the slower path through law school, no learning to allow his friends to see his faults and to live in the present rather than five years in the future. The plot beats are present, but the emotional tethers that tie them to character have been severed.
Not only are the dual plotlines of the movie not connected to Alex's character arc, but they are also largely unconnected to each other. In the book, when Alex tells his mom about Henry, Madame President freaks that Alex's personal life could affect her campaign and makes Alex sign papers stating that he hasn't used any campaign funds for personal reasons. It's clinical and uncomfortable, and further shows how Alex's relationship complicates the campaign and the campaign complicates Alex's relationship. In the movie, when Alex comes out to his mom, she smiles and orders pizza. Disconnecting the plot threads from each other doesn't just unmoor the journey. It also drastically lowers the stakes.
The big secret to storytelling is that effective stories are only really ever about one thing. Macbeth is about lust for political power. You could say it is also about a marriage falling apart, but the reason the marriage is falling apart is because of the lust for political power. Even if a lot is going on in a story, all the elements must point in the same direction, must support the same argument. Otherwise, the story will feel muddled and crowded, or some elements or storylines will feel much less engaging than others. Red, White & Royal Blue: The Movie adapted two separate storylines without considering what the unifying journey was that tied them together.
Red, White & Royal Blue isn't a bad movie, far from it. And I really appreciate all the plot details they adhered to. But when it comes to adaptations, it's not enough to transfer elements from the page to the screen. You also need to understand what purpose those elements served. If the elements are present, but their reason for being is not, the final product is going to feel hollow.