OK, I'll bite.
I'm a researcher myself. I study neurobiology or how the nervous systems of animal's work. My specialty is electrophysiology, which basically means directly measuring a neuron's electrical activity. My niche is neuroethology, or how do these electrical signals give rise to behavior in animals. In order to even have a chance at studying these things we first had to learn how to even record from such cells. That started in the 50's but progressed really slowly. We were first recording from one cell per animal at a time, and only from animals with neurons that were several hundred microns to a millimeter in diameter (think large Aplysia). In this early era too, all analysis was done by hand and signals were captured by triggering oscilloscopes and taking pictures of the screen with a polaroid camera. Once that was done, it took a long time to develop things like analytical and mathematical techniques to analyze all of this data. Nowadays, we can record from hundreds of cells at once, but determining what all of that data means is another feat. Basically, my small field took decades to develop because electrical recording methods had to come online, microscopy had to advanced, genetic approaches became readily available, computing became more powerful, and human ingenuity also developed. None of it happened at once (and it's still ongoing).
Studying coral diseases first requires being able to EASILY cultivate them in a lab setting so that you can isolate single variables. It needs to be so simple that graduate students and post-doctoral researchers are never limited by corals to study and they can focus on the questions at hand. How long ago did that happen? Probably 30 years ago we would never have had a chance to run a real coral lab. Now some Universities are attempting it. But still, probably less than a decade of real laboratory research on the topic. Then you need the molecular and genetic revolutions to come into play so that you can determine what is really happening to the animal under attack. Few coral genomes have been sequenced and I doubt RNA or proteome libraries have really been constructed for any substantial amount of corals. Finally as Bill mentioned, if we think that climatology has an impact on these diseases, well that field needed to mature as well (think how far satellite technology has come in the last 50 years).
Honestly, as someone who's devoted their entire life to lab sciences I can tell you it's not surprising at all that these things take time. Lots of time. Scientific progress is best measured in decades at least and never years. Also, we are at an interesting point now where things are actually moving even faster thanks to technology (computing and such) and the molecular revolution. You have no idea how much slower things progressed 500 years ago.
FB
Oh and yeah, cancer is a bad analogy. Most of your medical treatments for the time being are based on trial and error with clinical trials validating what works and what doesn't. Most drugs are the products of chemical screens meaning thousands of candidates are tested, and a couple appear promising. It's no where near as if we new exactly what we were targeting and then developed one drug to do it. Some drugs that work and help with chronic diseases are even discovered by accident as off label use or don't even have an known target. Yes science is both simultaneously amazing and utterly disappointing at times.
