Some games were added mostly to bring that game's bugs to attention and to try to find someone who wanted to tackle fixing those problems. When noone surfaces and people keep complaining the game doesn't work, mameDev removes them. (This has been repeated a bunch of times.) IIRC, die hard was one of those games.
Other games are removed when it's found that it wasn't any different from another version already in mame, or that it wasn't ever "found in the wild" (except for official prototypes). Other's aren't really removed, but just renamed; rom managers can find and handle that.
The "too new" games recently removed (mentioned in prior post) will be added in January releases. "Too new" is any game that's not over 2 years old at the first of the current year, or the game manufacturer contacted mameDev and and told them "Our game is too new to be included in mame" (or somewhat to that effect).
FWIW, some people are now mis-stating that the games now have to be 3 years old, but that would only be true if games were released only on the first of each year (or there was no such thing as 2.25 years old).
Example 1: lets say a game was released Dec 31 2001, today is Sep 24 2003; 2003 - 2001 = 2 years, but 12/31/2001 - 9/24/2003 < 2 years (~1.75), too soon even by the 2 years or older stance.
Example 2: current year is 2003, so 2003 - 2 = 2001; games must be released before 2001 to be added to a mame released in 2003. It's too hard for MameDev to check what (sometimes debatable) month a game came out, so the position is "assume all games are released after the first of a year, and only check if game age > 2 years on the first of each year." Sure, some game will need to wait until they're 3 years old, but the average would be 2.5 years (assuming games are released constantly throughout the year), which is less than 3 years old.