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Why are Muslims so insistent on burial rather than cremation?
Literal
Why, Muslim [genitive-の] people [topic-は], burial [to-に] insist [nominalizer-のか].
A cultural question posed with genuine curiosity. こだわる ('to stick to, to be particular about, to insist on') takes its target with に — ~にこだわる = 'to insist on X,' 'to be particular about X.' Common uses: 味にこだわる (be particular about taste), 品質にこだわる (insist on quality). 土葬 ('earth burial,' i.e., inhumation) contrasts with 火葬 ('fire burial,' i.e., cremation), which is the standard practice in modern Japan. The question gestures at a difference in funeral practices between Islamic tradition (burial) and contemporary Japanese custom (overwhelmingly cremation).