Because you're not translating 'цей', which is 'this'. Your version would just be 'Хто хоче їхати на фестиваль?'
I tend to disagree. Once it is decided which festival to go to, you refer to it as THE festival. You use THIS festival to introduce an opposition to THAT one.
I wrote 'drive' instead of 'go' and it didn't accept. Strange, because 'go' will be 'іти', but 'їхати' means 'drive'.