The missing explanation is that letter 'C' was always pronounced as the modern English 'K' in classical Latin, while K itself was actually a redundant letter. A Greek word with the letter Kappa borrowed into Latin, in Roman times, would have always been spelled with a C. Later, in Vulgar Latin and by extension in French and English, the pronunciation of C became corrupted and was pronounced as a 'S' or a 'CH' when it came before vowels 'E' and 'I'.
 
Therefore the objectively correct spelling (by Latin standards) would be with a C, and the fact that an alternate K-spelling exists shows that the word is a modernism and doesn't hail from Roman times. By modern English standards, both spellings are equivalent.
 
What I'm trying to say is that when Pearson first used the spelling "heteroskedasticity", he made a judgement call to intentionally go against the norm (for his own subjective reasons) and spell it with a K (according to Nick Cox's answer, he did the same with "Ekonometrika"). There is no linguistic motive behind this spelling other than the fact (perhaps) that he knew that these words would have once been pronounced with a 'k' and found it aesthetically more pleasing to spell it so.