Despite the similarity, because of occasional word order change, the modern Serbian sounds awkward. It doesn't flow as easily as Church Salvonic. And on some occasions even the meter is off.
So, just as we can't change Bethoven's 9th by adding words and changing tones, the same goes for the liturgy. It must be done in its original form to capture the mood for which ti was intended and to flow artistically and gracefully.
We all understand that things are not freely interchangeable (not every woman can be a substitute for one's wife, and not and old man can be your grandfather). These creations, including art are one of a kind and their mold is broken when they are made. Thus changes are merely bad imitations, and there is nothing worse than an imitation other than a bad imitation.
When you say "Church" Slavonic, is that the same as "Old" Slavonic?