why was Darwin's theory of evolution more powerful and useful than the proposals of the other scientists?

Because Darwin visited plants and animals in their own environment and brought back proof. 4 example: he hypothesized that the shape of a sparrows beak was due to the environment for which it lived in and its main food source. he brought back examples of sparrows from different parts of the same island that proved his theory.

none of the other scientists traveled such distances, therefore, they were guessing wrong ...