It’s this. Professional fighting forces.

Its absence implies laxity in command structures, which makes battle strategy nigh on impossible. There’s also a bit of a pre-Islam legacy of tribal smash n grab light cavalry fighting styles in play, but that’s also a part of under developed military professionalism.

For the initial crusades in the 11thC western forces were superior siege experienced, paid soldiers. Under Saladin in the 12thC, Muslim soldiery started getting its act together to more efficiently fight the western military machine.

Islamic forces and battle strategies would continue to evolve over the coming centuries to meet and better foreign invaders - and each other. See 1453 Mehmed II siege and conquest of Constantinople.