Too bad the Crusaders gave up so easily. The world would be a better place if they had wiped this cancer off the face of the Earth long ago.
There was nothing to fight for. Materially, that is. Wars are expensive, and way more expensive back in the pre-modern era, relative to productivity. Wars used to routinely kill off huge chunks of the population due to famine (and malnutrition-driven disease) resulting from war taxes imposed by each of the warring parties as they played musical chairs with territorial boundaries. Prior to an American figuring out industrial uses (other than the ancient one featured in the Aladdin tale) for the black crud that used to lie in pools on the surface, much of the Middle East was basically wasteland. Saudi Arabia had a population of 1.5m in 1900 (compared to 28m today) because, without oil revenues, that's all the country could support.
Europe had plenty of rain and fertile land. There was no reason other than piety for pushing into the Middle East. The Crusader kings spent huge fortunes in their quest. Frederick Barbarossa died on the way there. All-in-all, they spent more money than they could afford there, but learned a lot about fortifications, weaponry and military tactics, knowledge that would stand them in good stead during future Muslim incursions. And by not placing the cream of European chivalry in the Middle East, European leaders spared themselves the full weight of the Mongol war of annihilation that was to hit the region in the 13th century.