Ceres was the first asteroid/dwarf planet to be discovered. This was on January 1, 1801. Asteroids are hard to spot, so it made sense that the biggest one was found first. As telescopes got better, the number of discoveries increased.
But never mind the telescopes, nothing better than having a probe orbiting Ceres. Scientists and the public were intrigued by the mysterious bright spots Dawn was showing in photographs.
What did people think they were? - lots of people made their guesses. Who was right?
I'd say those that went for "salt deposits" were closest. It's not table salt, but a mineral salt, that's responsible. It's the sort of mineral salt left behind when ice sublimes (goes from solid to gas without melting). The fact that these deposits are found all around Ceres strongly suggests a layer of briny ice under the surface.