It's not that it was unknown and non-obvious - it's that as each successive stent was shown not be an effective treatment, cardiologists continued to operate for decades, confident that the next "new! improved!" version would work, even though none of the previous ones had.
The reasons for this are a combination of the difficulty of publishing neutral or negative results & surgeons' entirely natural inclination to do something rather than nothing, even if nothing is actually the right thing to do, because operating is what they've trained their entire lives to do.
The same thing happened with extreme chemotherapy followed by bone marrow transplant as treatment for many cancers in the 80s: something like a $billion was wasted on treatments that didn't work. The reasons why it took so long to demonstrate that that particular treatment was ineffective were not entirely the same - read "The Emperor of All Maladies" for the details - but shared the optimism of doctors & patients combined with a plausible theory, which is what ultimately makes for a potentially toxic combination.
The reasons for this are a combination of the difficulty of publishing neutral or negative results & surgeons' entirely natural inclination to do something rather than nothing, even if nothing is actually the right thing to do, because operating is what they've trained their entire lives to do.
The same thing happened with extreme chemotherapy followed by bone marrow transplant as treatment for many cancers in the 80s: something like a $billion was wasted on treatments that didn't work. The reasons why it took so long to demonstrate that that particular treatment was ineffective were not entirely the same - read "The Emperor of All Maladies" for the details - but shared the optimism of doctors & patients combined with a plausible theory, which is what ultimately makes for a potentially toxic combination.