Why should I use 由 here and not 被?
the problem may be that the A被B C and A由B C are superficially similar but grammatically completely different.  A由... seems like more a topic-comment structure for me, with A being the topic and "由B C" being the comment about the topic, not anything performed on the topic. 被 is conceptually linked to A - it is A having something done to it - whereas 由 is linked to B and C, and A is separate: again, a lot like topic/comment.

for example:


这个问题被他解决了. ： "this problem was solved by him."  straight up passive.
这个问题由他(来)解决 ：(as for) this problem, (it should be) him to solve it.


"这个问题由他解决了" completely fails the Google test, i.e. there's not a single usage example on the Internet, even though 被 is 100% ok.  This may be because the 由 construction is an example of a different kind of Chinese grammar structure, and therefore 由 is not like 叫/让/other stand-ins for 被.