I'm not sure that it's anything specifically about Mecca/Arabia: attacking any AI-capital will (almost) always be bloodier than attacking a 'normal' AI-city of the same size, because the AI (nearly) always stuffs its capital — and also its GWonder-towns — with (a lot) more defender-units than it puts into a 'normal' town (which normally only get 2-3 permanently stationed defenders).
Playing even just at Emperor-level, when I've built Embassies, I've sometimes seen up to 8-10 Spears in their cap. Once out of the Ancient Age, bombardment-units (preferably Trebs or better) become almost essential for sieging/capturing AI-capitals.