If Bitcoin addresses are truely random, then why are some more difficult to generate than others?
The 160-bit hash that is encoded in addresses is uniformly distributed ("truely random" as you call it), but the base58 encoded form is not. Some characters are more likely to occur at the start, for example.

To illustrate, consided the set of all integers between 0 and 1999. Even though each of those numbers is equally likely to be chosen, this is not true for its decimal representation. There, half of the numbers start with a 1!