Well, the realities of publishing these big stars means it isn't. Batman and Wonder Woman aren't really of the same "world" except when standing shoulder to shoulder in the Justice League. On TV, JL/JLU was the only game in town for them, so they never had to go back to their own corners of the DCU. But discounting all that real-world publishing stuff...
Frankly, I see more of a future for Bruce Wayne and Princess Diana than I do in Batman and Wonder Woman. Bruce might as well be a modern-day prince, the rich inheritor of a family fortune with incredible financial control of a city. He and Diana are a perfect fit, dancing and flirting the night away at some fancy reception. He's a perfect man and she a perfect woman, and both had to train to achieve that perfection (unlike Superman). But as superheroes, even the episodes that acknowledge their romance have them dealing with each other with gritted teeth. They might be too headstrong to work with each other. Still, they're both great interrogators and jet pilots. Maybe that's something they can build on!
Calling all WW/Batman 'shippers out there! Why do YOU think they make a great couple?