One of the reasons is, you don't get really good data on how something works until you start running clinical trials for it. It's all very time-consuming - having to plan how the trial is going to work, getting approval for it, finding subjects who meet the criteria (here, a specific type of cancer at a specific stage probably) and sites near them willing to work with you, manufacturing and shipping the treatments, and only then can you start gathering data. If it didn't work, you gotta start over, And it all costs a boatload of money too.