Why couples' therapy doesn't work?

First of all, it is not true that couples' therapy in general doesn't work. Quite the contrary really. What I do notice, though, is that there are differences between clients who come to individual therapy and those who come to couples’ therapy. These differences don't relate to differences in clients' character or personality but rather to the approach clients take to therapy. How clients approach and view therapy seems to have a decisive role to play in individual therapy often being more successful than couples' therapy. Which are the factors that negatively impact the outcome of couples' therapy?
1. Couples usually come to therapy too late. It often happens that couples seek help when their relationship is already so fraught that all they want is for the therapist to tell them that they should split up. Therapists can't and shouldn't take decisions on behalf of the couples. They can help them figure out whether the relationship is still worth saving or not. In the worst-case scenario, therapists can help couples end their relationship in a civilized manner, without too much anger, resentment, bitterness and vengefulness.
2. Only one partner in the relationship chooses the therapist. When that happens, it inevitably leads to the other partner feeling like the therapist is going to be on the side of the one who called and arranged for the session. Or, whoever chooses the therapist expects the therapist to be more understanding towards them.  Women frequently tend to expect a female therapist to be more on their side, and men expect a male therapist to side with them. Some sort of gender-based solidarity.
3. Expectations that therapists will singlehandedly resolve all problems. Couples like to transfer responsibility for the relationship onto the therapist. The underlying assumption is, "Here we are, we chose you, you're the expert, fix things." If couples don't want to save the relationship, they expect the therapist to tell them to end it. Unfortunately, that's not how therapy works. An important lesson of therapy is that couples start recognizing their own patterns that negatively impact the relationship, and learn how to solve problems together.
4. The demand that the therapist changes the other partner. It is a regular occurrence that couples come to therapy and one of the partners demands that the therapist does therapy with the other partner. So one of them comes to therapy in the role of an observer, willing to actively contribute to resolving the problems the other partner is causing, and the other in the role of a client who needs to change according to the dictate of the partner and in line with the therapist's interventions. I have already worked with couples where one walked out of the office saying, "Here you have him / her. Fix him / her. If I can in any way contribute to therapy with my opinions, let me know and I'll drop by." Again, we can't do couples' therapy with one partner only. Nor do we change the patterns of one partner alone.
5. Dropping out of therapy too soon. Most often, couples who don't want to work on the relationship and just want an expert to tell them it's better to call it quits terminate therapy too soon. They want to end it as soon as possible. Therapy is a process and sometimes it takes a long time. It's unrealistic to expect that problems which were accumulating for years or decades can be resolved in ten therapy sessions. If you feel you're not making any progress in therapy, change your therapist. Perhaps the problem lies in a bad therapeutic alliance.
Couples' therapy is most successful when partners begin therapy in time, before the relationship is broken beyond repair. It's also important that both partners assume responsibility for their respective contribution to relational problems. Both have to be willing to work on it. They need to change in therapy, both individually and as a couple. If you have a therapist with whom you get along, whom you trust and with whom you can openly discuss the most unpleasant issues, then there is no reason why therapy shouldn't work.