Too much time away from Diana. The wtiting was poor all throughout the silver and broze ages, and not bothering to update the romance at all during post crisis did serious damage. All the while we got saddled with the Legion of Substitue Steves or, even worse, pairing her up with Superman and Batman. So, as boring as hecan be, we are far better off with him than without until we finally get a female love intetest.
His main purpose is subverting gender tropes. Yeah the love interests in films are flat, but how many are male to the protagonist's female? Zip. Hes likeable enough now that I dont mind her being with him even if it can be kind of dull. The book still needs positive male characters, none of the others have stuck, so it might as well be him. He guards against worse options. As far as other male love interests go, we are not missing out on anything