Why the 2nd half of Invisible Man is not quite as good as the first half

The second half of Ralph Ellison's Invisible Man, in which the unnamed narrator becomes a political activist and a spokesman for a leftist group called The Brotherhood, is a little less interesting than the first part of the novel, for three reasons: first, quite frankly, it's a little less "black," that is, it's not so uniquely cast as the experience of a black man growing up in the deeply segregated South and moving to NYC to advance his life - the experiences of the IM in the leftist group are more universal and a little more abstract (not entirely, there are still issues of race, particularly as the IM confronts a new character, Ras the Exhorter, who advocates for a blacks-only leftist movement - we're very roughly seeing, in inchoate form, the cultural and political class that would dominate the next 20 years, between accomodationist, integrationist groups such as NAACP and SNCC and the black power movement - panthers, Malcolm X, et al); it's also far more polemical than the first half of the novel, which was more symbolic in mode: the black man mixing white paint, the explosion from too much built-up steam pressure; finally, it's a less of a peripatetic journey - from southern small town to college to nyc to a series of jobs and dwellings and more of an interior journey, as the IM is exposed to various political forces and oppositions, but all more or less w/in the same setting. That said, it's still a grand and capacious novel and very interesting even in this second half as we watch the various factions in The Brotherhood and in the leftist movements battle for the soul of the IM - who, at the point I have just reached, is "banished" from Harlem, in part for being too effective at rousing passions, and sent to a downtown office in Manhattan to continue his journey and to advance his knowledge.