Sorry, I edited in a second bit after because I realized I had only covered willful corruption and malice. But to extrapolate on it, basically, even (or perhaps especially) scientific theories can always be found to be wrong or incomplete after the fact; that's a fundamental part of science, after all, that we're always pursuing the truth. Let's say we've got our Lamarckian genius who comes into the picture based on his works on heritability; his experiments have been confirmed, his genius unquestioned, his political power within his field of study unparalleled. Now, suddenly we have this young upstart who suddenly pops in and says that there's this funny thing called genetics that explains all of his own work as well as quite a bit more. Our dear Lamarckian genius has made his life quite comfortable and has gotten quite used to power; now, he has a strong motive to suppress and/or discredit this upstart before his cozy nest egg can be pulled out from under him. Whether he succeeds or not in this, it's a dangerous incentive to create, and it's bad enough when the only thing at risk is tenure, one's name as a scientist, and one's potential mark on history; when one introduces real political power and financial benefit into the equation, it only becomes even stronger. Moreover, he may not even be doing it out of practical self-interest; he may have simply become set in his ways and, as you say, positively theological in his belief in his own theories.