For years, designers crafted sites and apps using Photoshop. This tool was never meant to do this type of work. First of all, everything produced in Photoshop is raster based, which means something designed at one size couldn't be scaled up if needed without losing quality. Imagine a small picture getting blown up to 5 times the size. Now imagine a button having that blurry, pixelated look. Secondly, it's a complex tool without a lot of feature sets for designers. But it was accessible, so people continued to use it.
Then Sketch came along. A powerful, vector based tool designed with UX/UI in mind, Sketch allows us to iterate fast and in vector, which means something stays sharp at 2px and 2000px. We can rapidly iterate, make multiple copies in the same view, and use a vast number of plugins to speed up our workflow. Sketch has been steadily growing for the past 5 years and is now the industry standard tool for UX/UI design.