He lost so Sasuke could see the truth, and so he could give him Amaterasu. -_-
so he wanted to give an evil psychopath even more power?
at ALL of it up and handicapped himself... he also just wanted to exctract the cursemark... he also saved Sasuke from death.
by beating him to the point where death was a possibility? also he didnt know tobis goals for sasuke. maybe he wants to manipulate him. maybe he wants to finish him off when sasukes weakened and depowered by itachi.
and the amaterasu trap is useless because itachi has no way of knowing if tobi will ever actually come there personally to talk to/fight sasuke. he has an army of subordinates after all.
itachi actually ensured that the only path left to sasuke was dying.
if he went back to konoha hed be executed for high treason by danzo.
if he doesnt, hes hunted down by konoha or akatsuki and killed.
sasukes sole option for saving himself was joining obitos side.