Final answer:
Wegener's concept of continental drift lacked a mechanism, leading to its initial rejection. It took advances in plate tectonics to provide the necessary explanatory framework for his observations, which then led to widespread acceptance of his ideas.
Step-by-step explanation:
Alfred Wegener's idea of continental drift was rejected by American geologists primarily because he was unable to provide a convincing mechanism that explained how the continents could move across the Earth's surface. Despite Wegener having presented several pieces of evidence, such as the jigsaw-puzzle fit of continents, matching fossils on different continents, comparable mountain ranges, and paleoclimate data, his hypothesis lacked the scientific explanation of what forces could cause such drift.
It wasn't until the 1960s, with advancements in geology, oceanography, and geophysics, that the theory of plate tectonics provided the missing mechanism that Wegener's hypothesis needed. This new understanding explained the movement of continents in terms of the Earth's lithospheric plates moving over the semi-fluid asthenosphere. Once this mechanism was accepted, the continental drift hypothesis quickly became a foundational aspect of the dynamic Earth theory.