I don't know the exact cause colonoscopies are not performing as thought, but for breast cancer it's quite interesting - basically treatment was improving at the exact same time early screening was being heavily promoted. And so years of survival continued to increase in a strongly proportional way to early screenings.
Naturally people assumed the correlation was causal, but it turns out screening mammograms are only marginally effective and that women are significantly more likely to receive unnecessary treatment (treating small tumors that would not have developed into large malignant tumors) than to receive beneficial treatment. [2]
If screening provides someone with piece of mind, then more power to them, but you're unlikely to meaningfully extend your life.