Developed by Game Freak, and published by The Pokemon Company, Pokemon Emerald was released for the Game Boy Advance in 2005.
Pokemon Emerald is seen as a combination of Pokemon Ruby and Pokemon Sapphire (after all, it is possible to catch both, Kyogre and Groudon in Pokemon Emerald). However, after the player defeats the Pokemon League, the game does not finish. After finishing the game, players get access to the Battle Frontier, a location where powerful trainers can battle one another (each "gym" in the area has a certain stipulation).
Pokemon Ruby and Pokemon Sapphire were remastered for the 3DS as Pokemon Omega Ruby and Pokemon Alpha Sapphire respectively. So, what happened to Pokemon Emerald? Why didn't it get a remaster? Seeing as it is a combination of both games, it only makes sense as it belongs to the same "family" and the same generation.