In Mahabharata, even Arjuna could not get heaven. How come Kaurava got heaven even after doing wrong things?
Well, you know, whenever people die they say, "Swargvaas ho gaya", means he has gone to heaven. This is one way to tell that someone has departed from this world. Not that they are all gone to heaven, but it is a term used.
'May their soul rest in peace' or 'Rest in heaven', is a general word. It is not that Kauravas were better than Pandavas, nothing of that sort. When they wrote this, it was written in a poetical form, so you should not take it so seriously. Otherwise, you will feel that Kauravas also went to heaven, then we might also go to heaven after doing wrong things.