Plate tectonics theory took decades to be accepted because

Because the main mechanism was not explained by Alfred Wegener

Explanation:

Plate tectonics was initially known as the continental drift theory and it was introduced by Mr. Alfred Wegener. He described that the continents moved over the large ocean from one place to another. He contributed many pieces of evidence in support of this theory, such as similar fossil assemblages, similar rock types that are found in distant continents, and matching of the continental margins.

But he was not able to explain the main mechanism behind this continental motion, due to which this theory was initially not accepted.

Later, this theory was modified and named as the plate tectonic theory which was widely accepted in the 1960s, where they found that the main mechanism for the plate motion is the convection current that forms in the mantle.

As Wegener could not provide a mechanism explaining how continents could drift.

Explanation:

The plate tectonic theory took time to get recognition and in the late 1960 was accepted as theory though it was published earlier in 1912, as the reason for the incomplete and irrelevant evidences given by the Alfred Wegener and thus his theory was criticized on various grounds and hence with a proper explanation of the movement of the plates was not accepted readily. After the knowledge of the seafloor spreading and the paleomagnetism studies made it eminent that the seafloor actually spreads and moves every year and that older rocks are located far away for the ridges and the newer ones are located nearby.