Answer: Many reasons but mostly Geography and tactical genius from some muslim commanders (Saladin).Because the crusader states were far from Europe and thus hard to defend (like far provinces in the Roman Empire).Don't forget most European forces went back to their homelands of Europe after a Crusade, and often the Muslim forces attacked while most crusaders were away. It was a catch and find game the Muslims finally won due to their strategic advantage. Although they never successfully invaded Europe.
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