It basically falls into three categories:
1. Games with licensing/legal issues
-Anything with licensed music, cars, brands, clothes, etc. that has expired
-Anything IP related made by a now defunct developer/publisher that wasn't bought by another (THQ)
-Anything that was directly involved in a legal case (Too Human, and I know KoA: Reckoning is still purchasable but adding a new marketplace to buy it from might be a no no)
2. Games made obsolete by another entry
-Sports titles
-HD Remasters/Ports (mainly Capcom at this point)
3. Games that aren't coming for not entirely understood reasons
-Publishers/Developers who, for whatever reason, don't want their games made BC. There's no legal situation holding the games back, just people and their own politics. As of now, I feel Koei Tecmo is a great example of this category. I cant help but feel there are several titles that haven't been made BC and don't have an actual acceptable reason as to why. I understand all the games need testing, but there are some odd omissions even with that accounted for.