The only reason I can think of is that LCS wouldn't be entertaining to watch if someone gets banned out and be forced onto a champion they're unable to make flashy plays with. Spectators want to see fluid movement, not clunky hesitation - that's why we're still at 6. Feel free to disagree, but that seems to be the obvious answer to me.
Edit: If it's going to be an issue for the pro scene, they can keep 6 while we get 10. Even 8 would be fine, we just need more. There are too many champions that aren't balanced and/or aren't fun to play against that we want to get rid of.
Edit 2: Another thing I wanted to add since I see the argument of "my mains could all get banned" come up, is that you likely aren't going to be banned out every single game. I mean, if you can only play FotM champions, you're likely going to be banned out, and that's your fault if you don't have something else to fall back on, so you should play champs outside of that group.
In all seriousness though, what are the chances that a single role is going to be completely targetted? What would happen 10 support bans go through champ select? There are still support options, same for ADC, same for jungle, same for top, and same for mid if they're targeted. Additionally, if you are concerned about losing because you're a one trick pony, and are afraid the extra bans could be used on you once the problem champs are taken care of, then sorry, you deserve the loss for having a champion pool with the depth of a puddle.