The narrator in 2001 knows everything. He knows what the slab is, what's wrong with Hal, what the aliens are doing and why, and what David Bowman is thinking when he's a super-baby. The narrator often tells you stuff before you really need to know it—so, for example, you learn that the Star Gate is a Star Gate before Bowman falls through it, somewhat ruining the surprise element.
So why does the novel tell you everything instantly and all the time, rather than letting you discover what the aliens are like for yourself with Bowman (for example)? The answer is that the novel doesn't really care about suspense or plot. It wants to give you the feeling of awesome distances and nifty new knowledge. It wants to give you the feeling of being an omniscient super-evolved space baby—and how better to do that than making you omniscient? In the novel, you see across time and space and into the brains of human and alien alike. What's a little loss of suspense compared to that?