The main reason given, aside from steering clear of non-Catholic religiosity, is:

The reservation of the ashes of the departed in a sacred place ensures that they are not excluded from the prayers and remembrance of their family or the Christian community. It prevents the faithful departed from being forgotten, or their remains from being shown a lack of respect, which eventuality is possible, most especially once the immediately subsequent generation has too passed away. Also it prevents any unfitting or superstitious practices. [vatican.va]

Guardian appears to have missed that, alongside utterly omitting a link to the primary document, as newspapers do way too often these days.

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