I feel like it makes sense to have new champs be disabled for ranked for a week or so after they come out, as to not throw a wrench into matchmaking. If they are underpowered, the players and teammates of the champion will take a hit to their MMR uncharacteristically, and vice versa for overpowered.
To give people something to bitch about - that's what this game is all about folks.
Answer: Sales
Like releasing a new racecar but saying noone can race it for awhile just drive it for practice xD
I actually agree, not only should they be disabled for the fact they may be too overpowered or underpowered but also you see way too many people "trying" the new champs on Ranked. Pretty lame in my opinion so I do think there should be a grace period where you must sit and learn the champion.
I guess it could also be debated that if they are overpowered or underpowered then you probably should of been reading up on the new champion and either banning them or practicing them yourself.
Whatever the case may be, Riot should have a system for when you own a champion that you should reach a number of games with the specific champion before you may play Ranked with them. Of course you would need to reach specific games with at least 5 different champions but that would ensure we do not have bozos flooding Ranked with new champions that they think are cool or op and get rid of those guys who have one good game on Draft and decide that they have the skill with a champ to take on Ranked.
Just some thoughts.
Boost sales no doubt.
In Dota, newly added champions are unavailable in competitive play for several months.
riot want people to purchase using that 7800 ip instead of 6300 disabling = less 7800 purchase less 7800 = more ip for other champs more ip for other champs = less rp purchases on champs = less $$$ for rito.
There are people who play the champions on PBE. Plus the lack of knowledge of a champion plays both ways. We used to be able to play champions on free week rotation in ranked, and that was cool. ("must be jax free week")
There's a faster and more reliable feedback loop on champion design when people are gaining or losing ladder points over it.
They should be disabled from rank on release so in the case of champions such as 
, game breaking bugs (such as that one that he had that let him make turrets stop shooting) can be sorted out.
Because people will play the champs on the PBE then play them when launched. You act like people either play the game or the PBE. Some people do both.
The simple reason is what many who play ranked forget this is still a game its **played for FUN its not the pro series or a military simulator **
That is why there are no separate rules for normal and ranked and why they allow new champs
All ranked is is normal's with better stats and minor prizes
