One reason is that "bloody" "has long had taboo status, and for many speakers constituted the strongest expletive available... Following the original use in England, Scotland, and Ireland, the sense spread to most other parts of the English-speaking world, with the notable exception of the United States, where it has apparently only ever achieved limited currency, e.g. among sailors during the 19th cent." (I'm quoting the OED, which I cannot link.)
So, "sanguinary" is a useful word for avoiding offense to those who take offense, and the OED even officially defines "sanguinary" — at definition #4, slang — as "a jocular euphemism for bloody adj., n., and adv., in reports of vulgar speech." Examples:
1800 S. T. Coleridge Coll. Lett. (1956) I. 564 This Extract breathed the spirit of the most foul & sanguinary Aristocracy—& depend upon it, Sheridan is a thorough-paced bad man!I'm not suggesting that Woodrow Wilson, in his original Armistice Day proclamation, intended to attach the suggestion of an obscenity to "war" — the noun modified by "sanguinary" — though it is common enough to call war an obscenity.
1890 R. Kipling in Macmillan's Mag. LXI. 155/1 This is sanguinary. This is unusual sanguinary. Sort o' mad country....
1910 G. B. Shaw Lett. to Granville Barker (1956) 168 The inhabitants raise up their voices and call one another sanguinary liars.
The first meaning for "sanguinary" is "Attended by bloodshed; characterized by slaughter; bloody," and the second is "Bloodthirsty; delighting in carnage." The first meaning for "bloody" is "Containing blood; composed or consisting of blood; resembling blood," which, interestingly, is less emotive than the original meaning of "sanguinary." So "sanguinary" can be considered more apt — quite aside from any desire to avoid a frisson of obscenity.