Not sure about Iscariot, but iirc Christ is a title, not a name. It’s actually Jesus the Christ, but that takes slightly more effort to say and languages often develop the habit of slowly streamlining themselves.

So it kind of became a quasi-name*~ because of how much easier it is to say as one.

*~or maybe ‘ascendant title’ would be more accurate? I feel like there’s a specific word for this that I cant seem to remember atm

Edit: the immense volume of translations made of the original text (and then ofc the translations of translations of translations etc) definitely play a role in Christ’s transformed usage, as well