Why doesn't Riot let you directly choose the role and champion you want to play?

Well concerning champion, because of ban phase. As for role, thats how it is , if everyone would be allowed to choose just 1 role, queue times would be HUGEEEE. U would have 100000 mid laners in queue and 1000 supports.

did you mean: teambuilder

Queue times, and draft pick. They had teambuilder around for over a year and got plenty of data on this exact sort of queue time testing and came to the conclusion that it takes far too long to find games for people that are similar skill levels when everyone picks their exact lane and champion. Also, there needs to be a draft system in place for ranked - and new champ select was initially concepted and begged for by the community as 'team builder for ranked'.

The compromise we got with new champ select allows players to never get their 3 worst roles, eliminates any arguing over who is going where, and allows for a draft pick phase, while also reducing queue times from what teambuilder was like.

because they removed team builder. Also, the long Q times could be fized by adding a third role, more people would put supp on it and it would mean less chance of ending up as a supp (the third role would be optional but yeah).

can you see my custom item? I've made an icon for it! http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/buOIBEzA-the-phoenixs-talon-icon-made-by-me-3