You are making the mistake of thinking that monkeys are inferior to extinct species of early humans. Monkeys are well adapted species for living in the environments where they reside. There is no objective trait that will determine what species survives natural selection, such as "being smarter" like you have mistakenly assumed.

Earlier homonids would have been proficient in tool use and maybe even communication, but if a new variant would show up that is more proficient in exactly those areas that made the older ones succesful, and provided that he survives and procreates, his descendents will graduately displace the older variants.

Monkeys didn't suffer from the same level of competition and thus lot's of monkey species remain, each well adapted to their respective environments.