Incredibly hard to find sales for it, almost only collectors own one, and they actually most of the time just bought the strangifier and applied it to a tradable head they already owned

no premium search feature -> track sales through outpost, bazaar

lazy suggesters -> not willing to track sales through outpost, bazaar

more likely to be bought as unique, then strangified -> sales are rare anyway

outpost doesn't have tradeable/non-tradeable paramater -> the rare sales are hard to find, if they do exist, by the lazy suggesters

HHH set -> non-set sales chance is reduced