The simple answer for why the Rotherham sex abuse scandal happened- and why similar cases have occurred in other towns and cities in the UK- is that no one knows for sure although educated guesses can be made.
This is because race has become such a taboo that the problem was not officially acknowledged for a decade and even now most* respectable media outlets start from the assumption that "it is has nothing to do with race" and then work backwards to support their predetermined conclusion.
The reasons for the taboo about prominently discussing ethnic issues exists for reasons that are somewhat understandable. No one doubts that in the past sweeping generalisations about racial groups have been employed by demagogues to whip up racial pogroms or justify treating people as second class citizens because of who they are.
There are more cynical reasons too- politicians wanting votes from ethnic minorities and "community leaders" wanting grants but the aversion to talking about race is sincere in most cases.
There are two kinds of problems with this approach, it is morally wrong and it doesn't work. The truth has a habit of coming out eventually. The strenuous efforts by Rotherham Council, South Yorkshire Police and others to conceal the fact that Pakistanigangs were systematically targeting vulnerable young girls has to put it mildly backfired in a spectacular manner.
Honesty is the only way of discussing race that has a chance of succeeding in the long term. In this case it has to be acknowledged that there is a specifically Pakistani (not "Asian" or "Muslim" but Pakistani) problem that needs to be identified and ended.
* Some media outlets deserve praise, the Times's Andrew Norfolk exposed the problem years ago but was ignored.