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Where do you get this definitive statement from?Kuhn's Structure of Scientific Revolutions was published in 1962. For over half a century since then, sociologists, historians, philosophers, and political scientists have been studying how modern science works, both on a small scale (in the lab) and on a large scale (as a society). Most of the results seem to confirm the basic idea set out by Kuhn, although many details remain controversial.
Rest assured, the boldness and tentativeness of scientific conjectures are not under any serious doubt. The part of Popper's model that later researchers have challenged is mostly about refutation and/or verification. For political and psychological reasons, as well as practical limitations, it is exceedingly difficult for scientists to conduct experiments that conclusively and unambiguously refute a hypothesis.
As a result, the bleeding edge of scientific research often follows fads rather than solid experimental evidence. Personal charisma, funding decisions, and political concerns dominate decision-making processes, and evidence is often so ambiguous at this stage that anyone can cite it to their advantage. It takes a while for this craziness to settle down.
In fields like theoretical physics where it is especially difficult to perform meaningful experiments, the time it takes for the dust to settle tends to be longer than in other fields. It can take decades, compared to months or years in other fields.
Only when a theory is no longer a fad and falls safely behind the cutting edge (i.e. it becomes "normal science"), do scientists finally get a chance to scrutinize it with solid experiments. Even then, politics can get in the way of proper refutation while the proponents of the theory occupy authoritative positions in academia. As Max Planck once said:
> "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."
(The above quote is often summarized as "Science advances one funeral at a time.")
Summary: A lot of conditions need to be just right in order for bold conjectures to be boldly refuted on the basis of solid experimental evidence. Bleeding-edge science often doesn't meet such conditions, due to both technical and human shortcomings.
Source: I have a Ph.D. in Science and Technology Studies (STS) [1], an umbrella term for interdisciplinary studies of science and technology in social, political, and historical contexts.
[1] https://en.wikipedia.org/wiki/Science,_technology_and_societ...