In the case of Darwin's finches,an ancestral finch species from the mainland arrived on the Galapagos Islands and soon developed into many new species via adaptive radiation.The finches did NOT undergo adaptive radiation back on the mainland.What is the most plausible biological explanation?

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When the founding population of finches reached the Galapagos Islands they would have encountered a variety of habitats and food sources but there would not have been any other species of birds competing for these resources. This would have resulted in multiple selective forces acting on the population and produces multiple species. On the mainland where there are many of competing species of birds the finches would have been adapted to a narrow set of environmental conditions.Bloom's

In the Case of Darwin's Finches,an Ancestral Finch Species from the Mainland