There are a few reasons why an achievement is no longer attainable. Sometimes, the multiplayer servers are shut down for older games, which results in certain online achievements becoming defunct. In some extreme cases, the achievements can no longer be acquired because the game can no longer be played, such as online-only Xbox Live Marketplace game 1 vs 100. As the game was only available during times set by Microsoft, and the game has been discontinued, these achievements can no longer be gained. Full House Poker has achievements that were only attainable during certain "seasons" of the game, like "Spring 2011."
These unattainable achievements, as well as other online-only achievements, are often used by Microsoft to detect accounts that are manipulating their Gamerscore.