The point is that mutations are simply failures of RNA to match the right pyrimidines and purines together during transcription. They're not programmable.
That would be difficult to manage. And the point is that they can all mutate equally, but a lot of mutations result in the cell dying or not reproducing.
The best thing to do would be to make a virus that exploits a part of a cell that changes slowly, such as something that is very difficult to change and get to work right. Certain parts of a cell, like receptors, can change, leaving the cell fully functional, but other parts would have to change in exactly the right way for them to still work. Target the latter, and it works.