One of the reason is because it's kinda prohibited to do so in Islam. Religious aside; giving, accepting, and shaking by right hand is one of the basic norm of Indonesian culture which even being taught to children in elementary school (civics subject), along with the others like "respect your elder". So, it's not okay to do them even if that someone is not a Moslem.

Hi Tim! The reason why we are encouraged to use our right hand instead of our left is because the right hand is consider to be impolite. Why is it impolite? it is because we always use our left hand to do unclean job, this include clean up your b*tt after doing number 2 :D Therefore, it is best to use your right hand when you handover money and gifts in Indonesia. However, if you REALLY have to use your left hand because your right is holding stuffs, you can use you left and said, "Maaf" (Sorry) and then you can pass the things on. I hope it's clear enough for you Tim. Sampai jumpa! Regards Zacky

Indonesia is a Muslim majority country. Do you know what the left hand is traditionally used for in Muslim countries? You can look up the details online but basically the left hand is considered unclean.