Why is the term "isn't it?" so predominant in Indian English?

As John Lawler comments above...

Indian English has made considerable changes from British English in idiomatic syntax and phrase formation. Isn't it? has been frozen into an idiom in India, and it no longer varies the auxiliary verb or the subject of the tag question.

I think OP is mistaken in thinking BrE innit as appended to statements is "Cockney". It's now quite widespread, particularly among younger and less educated speakers. But I'm pretty certain it originally arose within "second-generation Asians" (i.e. - people who were born in the UK of parents who came from Indian or Pakistan). Their parents were already using IE isn't it; they simply Anglicised it to innit.

Obviously those younger people who took the trouble to Anglicise a form like that (or adopt it early when they heard it) would disproportionately include those who were both "proud" of their parents and wished to integrate into mainstream British culture. That being a much-admired characteristic in multicultural Britain, young people from other non-Anglo-Saxon backgrounds (esp. Negroes and West Indians) were also quick to adopt it.

5-10 years ago I used to sometimes nag my son (now 25) when he started using innit (I thought it sounded "ignorant", particularly when used in contexts where I'd expect pluralised aren't they). Obviously my son took no notice of me, and now I've just got used to it because it's everywhere.

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I know it looks as if I'm banging on about the BrE "import", rather than addressing OP's specific question (how did it arise in IE in the first place?). But I think the underlying reasons for the uptake have much in common. The full-blown "tag question" form, requiring verb number/tense agreement, can be tiresomely "finicky". Probably part of the reason why mainstream speech went from is it not [so]? to isn't it[?] in the first place is that people actively seek informal usages that trample over traditional grammar.

My own feeling is it's slightly "traditionalist" to think of IE isn't it and BrE innit as "tag questions" at all. They're actually more like appending yeah[?] to a statement (or the stereotypical BrE toff's appending of what or what-what a century or two ago). Semantically and grammatically, the "question" element is long gone.

As an English speaker growing up in an area with a high number of Hindi speakers & going to school with Hindi & Urdu speakers, I noticed a constant switch between using "hai na" and "innit", the transitional "hainit" they drop H in "ai na" and the transitional drop H in "ai n'it" and the formal "isn't it" depending mostly on audience & accent.

To be honest, with a colloquial English accent, most of these variations blur together when spoken. I personally think that it is this aural similarity combined with the similarity of meaning that has driven this use of language.

I think the phrase is not only predominant in Indian English, I always hear it in British English too, we Filipinos always use it, but mostly just the translation of it. I think, through American Colonization, we FIlipinos also adopted it in our language.

Us Filipinos' Tagalog has hindi ba, contracted to 'di ba, colloquially used as diba, sometimes with or without a question mark, sometimes the contraction's first syllable is accented, sometimes it is not. The phrase literally means is it not, (re)translated back into English as isn't it.

So it left me thinking that if Filipinos got it from Americans and Americans got it from Britons, then it is reasonable that it originated from India... like tea. :)