Why is "soccer" pronounced with a hard "c"?

Solution 1:

The term soccer appears to derive from Assoc, short for Association, originally “scocca”, with a hard c:

1889, socca, later socker (1891), soccer (1895), originally university slang (with jocular formation -er (3)), from a shortened form of Assoc., abbreviation of association in Football Association (as opposed to Rugby football); compare rugger.

An unusual method of formation, but those who did it perhaps shied away from making a name out of the first three letters of Assoc.

- er :

suffix used to make jocular or familiar formations from common or proper names (soccer being one), first attested 1860s, English schoolboy slang, "Introduced from Rugby School into Oxford University slang, orig. at University College, in Michaelmas Term, 1875" [OED, with unusual precision].

(Etymonline)