What is an objective criteria of "being successful" for the type of species? If we compare humans vs cyanobacteria, then "total mass on Earth" and "the number of species" are both against us, humans ). Of course, you can say that humans are more "complicated", but unless defined it is poetry, not science. The only thing I can think of, is when our Sun fades humans (probably) will be able to escape to another stellar systems, while bacteria will die on Earth. However, evolution is not aware of the limited timespan of Sun so it could not be the cause. So while this subject looks intuitively simple (of course, simple evolves into something more complicated), on a second thought I don't see an solid underlying reason for it.