We study the isospin amplitudes in the exclusive B to D⁎D¯K decay process and fit the available D⁎0D¯0 invariant mass distributions near threshold. The analysis demonstrates that the production of the isospin triplet D⁎D¯ state is highly suppressed compared to the isospin singlet one. That explains why the Zc(3900) has not been found in B decays. In addition, the production of the negative charge-parity state might be further suppressed in the heavy quark limit. These two reasons which are based on the molecular assumption offer the first explanation why the Zc(3900) is absent in B decays. Further studies of the absence from both the experimental and the theoretical side is extremely important for understanding the nature of the X(3872) and the Zc(3900).