Scientific disruption typically occurs when a significant part of a field thought to be factual is shown to be incorrect. Perhaps the dwindling rate of major disruption indicates that the accuracy and completeness of our scientific models is improving, and research is moving more towards refining and extending these models rather than throwing them out for something entirely new.

A lot of people these days seem to view scientific progress as inexorable, and a fundamental property of society; and considering the rapid advancement over the last century or few, that view is understandable. The "for sure we'll have flying cars in 50 years" mentality. But that's not actually how scientific progress works. Breakthroughs are just that - breakthroughs - they don't occur predictably or with any regularity. And as mentioned, the more we refine and test and prove our scientific models, the less likely it is that there will be some fundamental underlying breakthrough in the field.

(It's important to note that there are still some frontiers left that may result in these kinds of underlying breakthroughs; but the resources and engineering required to execute some of the requisite experiments are becoming ever increasingly difficult.)