Not sure, but maybe that translates to "this is more good (more well?)" which sounds odd. While one may argue that that is what "better" means, no one really says that when the word "better" is available to convey that. Maybe it is the same with French.
I wrote "Ça c'est bien mieux" because "C'est bien mieux", to me, means "It is much better", and I needed something extra to emphasise the "this". Does that make sense? Could someone explain why "Ça c'est bien mieux" is incorrect, please?