The original Solaris novel had a different ending, a disillusioned reflection by Kelvin, which I found much better. It was probably not dramatic enough for a movie, too analytical.
Personally, I always found that the novel ends on a comparatively hopeful note, especially given the circumstances, the final word being cudów ("of miracles").
Aside from this, perhaps another reason why the first part of the movie looks and feels more promising than the second one is more trivial. As far as I know they experienced a budget shortage at some point of production.