Why is personal playstyle so huge in LCS?

For example Voyboy playing tons of assassin style champions and ones that can dive in. Many players play a set pool of champions that match their play style, and not much outside of that. In my mind shouldn't a pro bring the ability to play every style in his position. I feel that should be the standard in LCS, yet it isn't. Why are style specific players so common in LCS? And do teams pick them because of their styles?