I'm afraid I can't give you a definite answer but might be able to hazard a guess - apologies for the lengthy explanation that follows.
One reason may be that ISD is (at least in humans) a genetic disease and so is hereditary. It is caused by a recessive allele and, even then, only a small proportion of those that are homozygous recessive ever contract it.
To give a possible example, two rhinos - 'Emi' and her daughter 'Suci' - both died of ISD (and displayed similar symptoms) at the Cincinnati Zoo. But 'Suci's' father 'Ipuh' died of cancer aged 33 and neither of her brothers ('Andalas' and 'Harapan') exhibited any symptoms during their time in Cincinnati. All five animals were kept at the same zoo and so, presumably, were exposed to similar conditions and diets. Perhaps genetics could account for this disparity.
Similarly, it is possible that ZSL was lucky enough to acquire two animals that were not homozygous recessive and thus not susceptible to ISD - whatever their diets. I should stress though that this is based on an understanding of ISD in humans - I don't know whether it is the same in rhinos. This hypothesis is not original - Cincinnati Zoo has suggested something along similar lines - but is still unproven.