Because otherwise the default explanation for why there's no ISO standard for Cantonese romanization is the same as why there's no ISO standard for romanizing Burmese, or Malaysian Jawi etc. There's little demand internationally for such a standard.
And if the Hong Kong government's standard, or the Guangdong government's standard (this one presumably preferred on the Mainland) is officially blessed by ISO, would people currently using something else really switch just because of ISO?