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(in the sub continent perspective) what was wrong with arabic or persian that the ulama started using urdu? why didnt they insist that people learn arabic or persian in the same way that some insist everyone should learn urdu?

I think Urdu evolved over time. The early Muslim rulers of India were from Turkey/Central Asia and those who came with them either spoke Turkish or Farsi. I think eventually Urdu came into being because of local population of India spoke Hindi/Sanskrit. So the script (letters) was composed of Arabic/Farsi letter plus some Hindi sounding words. So thats why Urdu is spoken like Hindi but written like Arabic/Farsi.

Moving over to Arabic would not have worked in India as majority population was always Hindus, so Urdu - which was hybrid of Arabic/Farsi and Hindi, was a natural choice.