Victini is UU because its weaknesses are appaling, like the entire metagame will have a water, rock, ground, dark or ghost move. furthermore these weaknesses prevent it from using its ok defences. its attacking stats are nothing special, and V-create is all that makes it Better than other pokemon of its description. victini also packs a stealth rock weakness. it's niche does not even allow it to work in OU, as IMO choice scarf heatran will work better than scarf tini, due to it actualy resiting things, and CB darmatian packs all the power victini has under the sun without the extra weaknesses and with a better ability. finaly in OU victini has a OK base speed, but this metagame has sped up since 4th gen and victini is now outpaced by so many threats when unboosted, and weather teams speed boosts will make the scarf redundant.
thats a pretty comprahenssive list of why victini is UU, though it is suspect, due to its huge power V-Create. even if it is voted out of UU it seems unlikely it will be used in OU, except for the odd nutter who likes the design