Writing code in a file is faster than dragging events in an eventsheet, also, writing everything yourself gives you more flexibility, you wrote that code by yourself and you know how it works under the hood, and it's easier to debug it with a console (You can use the Browser in C2 but eh, not really interesting) ... Finally, C2 is geared towards beginners, most notions like floats, arrays, pointers, classes, functions are all hidden or even removed from the user's perspective, which makes it not better than other languages, which are consise and allow you to do everything with a limited syntax.
But that's all fine because C2 did what it was meant to do, it gave the ones with no programming experience the tools to build games, but it's design seems unappealing to professionals and that's why they don't like using it for big games, on the other side, when you get to know it, C2 is as powerful application that is completely suited to make big games, it's just different than the rest of the market, and people use that as an excuse to not use it