Licensing could be a problem for some older games, as I know already Nintendo is trying to deal with Microsoft for rights to the Rare Nintendo 64 games. Beyond having to deal with competitors, the fact that some of the old game companies no longer exist is another issue. Royalties are yet another potential problem depending on the agreements signed in the 80s on many of these games.

The other aspect is that from a sales point of view, releasing a slow trickle is likely to bring in more cash in the long run then releasing everything at once. With the trickle method, more people are aware of all the titles being released, and thus they might pick up one that they forgot the name to, but remember when they see it new. Compared to having people dig through a massive library just to find the games that topped their list back then.