yu yevon want to keep sin alive...forever...even after the dream of the fayth gone...(well if yu yevon manage to send it)...sin actually is a punishment for the spira citizens...the yevon teaching teach the citizens the thing they can do and otherwise...but the citizens somehow forgot and leave the teaching away as they fight to each other (zanarkand vs luca) if im not mistaken...yevon got mad and sin is the answer (yevon punish them)...but the dream of the fayth corrupted yevon plan...the longer they dream...the bigger the posibility for the dream to come true...as a result...sooner or later the dream will demolish sin and yu yevon itself...yu yevon cant let it happen by just watching it come true...so he uses sin as a shield to send the dream...somehow yu yevon is too late...because jecht accidently found a space to travel between the fayth dream to the real world...he together with auron and lord braska succeed to stop the sin for the coming 10 years (false hope)...after that tidus son of jecht come and this time he will become the only hope to stop the sin for not the coming 10 years but forever...after tidus beat yu yevon and the sin (lord braska final aeon and his own father)...the dream of the fayth automaticly gone because the fayth is gone...the fayth is gone because they knew that spira has reach the permanent calm as sin will never be exist again...just consider the fayth mission has accomplished...this bring the reason of why tidus is gone too...got it!