BECAUSE I'M EITHER TOO BUSY IN ANOTHER WINDOW DURING CHAMP SELECT (and the client does not pop up in front when it's my turn to ban) OR I THINK IT WILL AUTOMATICALLY LOCK IN MY CHOICE (when I'm clicking champs at random for fun)...
PLEASE. OMFG. I literally cried one time because I had to wait 30 minutes.
Times will be even shorter next patch, and Riot appears to have no intention of removing the lock-in requirement to make players pay attention to their screen and move the process along faster, so you should probably just lock in as soon as you've decided.
The issue where the client is not popping to front is known and will be addressed in a future patch. http://boards.na.leagueoflegends.com/en/c/riot-official/zJ4kqlHZ-the-future-of-new-champ-select?
