Because he DIDN'T have anything meaningful to do with the actual production of the film, as evidenced by it's radical departure in tone and style from I-III.
I don't know which movies you've seen, but the
First Class that I've seen returns to the tone and style of the first two films.
The Last Stand is the one that stands outs as different.
FC's fight scenes are dynamic, integrated and fast paced. I-III's fight scenes are stagey, ponderously edited and underpowered.
FCs plot moves right along with only a few nods to the "mutant question" subplot that don't bring the story to a screeching halt while the characters sit around navel gazing about how crappy life is because they are mutants, unlike I-III.
FC's characterizations sparkle with wit and humor where appropriate (Charles, et al), and harsh intensity where required (Magneto). This is in contrast to the very subdued cast of I-III.
FCs cinematography is expansive and visually interesting. I and II look cramped and claustrophobic. III tried to open itself up visually, but still relied heavily on relatively tight, static shots most of the time.