Or "Why is Ant-Man the last movie of Phase 2, rather than Avengers 2?"
The Marvel Cinematic Universe is the series of movies involving characters from Marvel comics and produced by Marvel Studio/Disney Studio. The series of movies is decomposed into phases, that are different steps in the studios' production plan.
Phase 1, entitled Avengers Assembled, is composed of all movies from Iron Man (2008) to The Avengers (2012). This phase relates the origins of each Avenger and how these mighty superheroes came to fight as a team.
The narrative purpose of Phase 2 is less clear to me. At first, I thought it was about Thanos' story arc but Thanos doesn't seem to appear in Ant-Man, the last movie of the Phase 2. Actually, Thanos will most likely be the main antagonist of Infinity War, which is a part of Phase 3.
Therefore, what story arc will be concluded at the end of Ant-Man? I believe it can be answered before the movie is released, as the purpose of Phase 1 was clear before the release of The Avengers. Marvel Studio may also have commented about the organization of the movies. In other words, what narrative elements unify the movie of Phase 2?