The tracks are missing for a very specific reason. Robin Williams was reluctant to do voicework for Disney, because he disagrees with their business practices. He was finally pursuaded to do "Alladin" but he specified that Disney was only allowed to use his work in specific ways. Well, as soon as Alladin was released, Disney immediately violated the contract they had made with Williams and exploited the "Genie" character, used the songs in commercials, etc. And so Williams sued for breach of contract and now any of his voice work is not available outside the movie itself.
I figured this out a little while ago, and it made me a little sad. Aladdin was, hands down, my favorite movie as a kid. I would watch it whenever my parents were arguing, and I would wish for genie to take me away from my life (lol). Seems pretty ridiculous now that I think about it. Still, Prince Ali was my favorite song from the soundtrack, and it's absence saddens me. I feel like having Robin Williams's songs stricken from the album dishonors his work. Just my honest opinion.
As far as I remember, Williams had it as part of all his contracts that his voice couldn't be used following his death of any circumstances. He didn't waht them using it however they wanted. Because they own the rights to his performance their only option would be to give up rights to the song and not profit from its existence. He actually did want it this way.