"in" is acceptable but "on" is definitely the more common expression in this context.  Why? there is no why; it's just one of those perverse idioms of English.  We can be on form but later in decline; you might be instructed to do something "when you are in receipt..." but "you should, on receipt, ...".  The "up" and "down" conventions are even worse: we cut down trees, but then chop them up; after dinner we wash up, but to get rid of the suds we rinse down.  I could go on, but the short answer is I don't know why.  I know copper is not the only material, but that is the common expression, especially when disambiguating against fibre-optic. The reason here is obvious - it is by far the most common metal used for cables.  I don't really care if the article title gets changed, but this is what I think it should be.