why is San Diego Didacopolis rather than Sanctus somethingus?

Who knows the various ways of the universal church? I'd always heard that San Diego (St James in English) is from Santiago, so why not Sanctus Jacobus? San Diego > Santiago (cf. Santiago de Campostella) by metanalysis (sant + Iago > san + Tiago (finally Diego). But, I see that St Didacus is the patron of San Diego, and that in Spanish, he is known as San Diego. The Latin name of San Diego is Didacopolis according to the Diocese website. I'll have to research it.

Seems I was right. Didacus is a back-translation of Diego (also Diaz) from Spanish into Latin. At least according to this Wikipedia article.

Ceci n'est pas un seing.