I already wrote it earlier. Because it was not published for a long time, until preparation for the attack is complete.
So, the merchant sees a clear sign that his transaction hasn't been seen by the network for a long period of time and he still thinks that the service/goods should be provided without waiting for much longer?
You claim that your design allows mergers of split subtangles. The merchant sees something like that. Your algo, described in the whitepaper, shows good confirmation score. Do you want merchants to analyze the DAG manually?