Why do soliton modes in polyacetylene have to appear at the edges of an open boundary chain?

Question

When calculating the presence of soliton or anti-soliton in the extreme dimerization polyacetylene SSH model, we say that in the case of open-boundary condition and odd number of atoms, we must have an edge soliton. But why is that the case? I understand that we must have a soliton, but why is the edge mode preferred over the bulk case?