​If Popper is so good and useful and true, why isn’t he more popular – especially among professional philosophers and scientists? Shouldn’t this count against him?

Conjecture: One can try to disagree with just a key part of Popper’s philosophy (like his take on the line of demarcation, or the paradox of tolerance or what democracy actually is) – and attempt to embrace the rest, but then one finds inconsistencies in their own thinking. It becomes clear to a casual reader that if I reject this key part of Popper's philosophy, then (logically) that entails rejecting that part and then that part and that part and so on. Soon I have to reject it all.
 
And I must reject it all if I reject some key part (like authoritative sources, or a critical method) because to do otherwise would be to change my worldview and thus so much of the received wisdom I have been taught or indoctrinated with. So I have a choice: embrace the change and indeed change the way I think about almost everything in my intellectual life (and even about what I regard as common sense) – and this might be painful (in truth, to a Popperian, it’s fun – or it can be learned to be fun).

Or I can simply reject Popper.
 
Embracing the critical rationalist worldview/Popperian-style philosophy is deeply psychologically destabilising at first. It means viewing reality in a new way and thus removing oneself, in a sense, from one’s intellectual peers. Popper is not like other philosophers. I can reject elements of Descartes (let’s say his “proof” of the existence of God) – but accept his cogito (“I think therefore I am”) and feel no great destabilisation in my thinking. Leibnitz’s “monadology” whether I embrace or reject it, would seem to have no great bearing upon how I think deeply about each and every issue. Perhaps there is a fundamental indivisible particle. Perhaps not. Perhaps there is an intelligent creator. Perhaps not. Much of this is disconnected, to some extent, from the problems I am interested in my life anyway.
 
But Popper and Deutsch change (and challenge!) my actual thinking – moment to moment. And on topics near to me. I cannot be sure of anything – but I know quite a lot. I can improve my thinking and myself. I can make progress. I am self reliant, but I can cooperate. There is no authority I need to turn to, but I can criticise any who claim to be a source of truth and thereby come closer to the truth myself. No one has the final answer (including me) and “we are all equal in our infinite ignorance”. And that last one can give one a real sense of vertigo upon first really comprehending it. And not all people find vertigo - that sense of falling - fun. But we can all learn to. Yet few people, as a proportion of the whole, ever choose to go skydiving despite the recommendations of almost everyone who has.
 
Philosophy - and Popperian philosophy in particular - goes right to the deepest parts of one's psychology and how they frame their thinking on any topic worth thinking about. It requires a shift in gears, which many (not all!) find unpleasant - at least at first. Like skydiving or roller coaster riding. Should the unpopularity of any fun thing count against it? Of course not. Most people simply do not know what they don't know. It does not count against skydiving that it's unpopular. Nor anything else people find gives richness and purpose to their lives but which is poorly subscribed.

​And if one has spent a career defending a mainstream thesis, or a thesis that has merely adapted some ideas within a mainstream framework, it is unsurprising then, that one would be reluctant to relinquish all that framing for fear of the mental pain and anguish which might accompany it.
 
Unless, of course, that person is truly, to their core, a Popperian. In which case that kind of relinquishing is one of the most joyful parts of life.