As I understand the instability of mRNA in the general cell background cytoplasm is because of the presence of enzymatic molecules (produced by the cell) that attract and then break down (lyse) any mRNA that floats into their 'jaws'. Loads of different things get broken-down by this kind of background presence of some enzyme, and since mRNA is used by the cell itself, maybe not too surprising that it has a cleanup mechanism in-place already?