The Docile Grasshopper Turns Into a Swarming, Ravenous Locust

People thought that the grasshopper and locust were two completely different species. The docile grasshopper turns into a swarming, ravenous locust when drought happens. Why did it literally take centuries to figure out that these two creatures were actually the same creature? And why, even after the theory was proposed, did scientists resist it? Maybe it's because we can't see that in ourselves.