New Champs and Reworks on PBE BANNED

I'm just curious as to why new champions and champion reworks are bannable on PBE? Does that not make it more difficult to play test them? Every game I've been in has seen the new champion banned, making it difficult for me to play as, with or against her. I've so far only been able to see her once in a bot match I created, making it rather difficult to see her with actual people where I can see how she works, what doesn't work on her, etc.