sharingan doesn't just go around and copy the technique.
let me explain it.
sharingan sees through the technique. how it's done?, which seals are necessary? etc.
but not everyone can do the thing, that he knows how to be done.
for example, wen jiraiya taught rasengan to naruto he first demonstrated it on a fodder ninja.
then he told him that rasengan is about "releasing chakra, then rotating it, then concentrating it."
but if it was sasuke while sharingan activated, jiraiya wouldn't have to explain that. with the first demonstration sasuke would understand that it's about "releasing, rotating and concentrating." however, sasuke doesn't know how to release chakra from his palms, or how to rotate them. so he can't use rasengan without training
and that's the reason why only kakashi has achieved this feat. even before obtaining sharingan, kakashi was already well-versed in all fields of ninjutsu. sealing, elemental ninjutsu etc. and after obtaining sharingan he worked on using other natures. such as earth and water. (maybe fire too).
since he could do lightning, earth, water, sealing and special jutsus that require certain skills, such as rasengan, the only thing kakashi required was to know how to do a technique. and there comes the sharingan. with it he could see how a technique is done, and since he has the skills he was able to "copy" the technique.
but since the uchiha depend mostly on their eyes, they didn't work enough on other skills. so even though they could see how a technique is done, they didn't have the skills to do it. thus they couldn't "copy" the technique