Because it makes no sense? For all the uses of 'mainland China', it would be simpler to just say 'China', with no loss of clarity. Adding pointless words just for the sake of it, especially when it seems that the author(s) don't mean what the word says, is bad writing.
Hong Kong is inside China but not governed directly by PRC. Most non-Chinese citizens need a visa to enter "Mainland China" but not Hong Kong. Most Chinese people need a permit to enter Hong Kong. So, yes, the distinction is useful.