The theory of evolution does not predict that there will be multiple species of humans living contemporaneously. Evolution is a gradual process that occurs over long periods of time, and it does not necessarily result in multiple species of a particular lineage existing at the same time.
The idea that multiple species of humans should exist if evolution were true may stem from a misunderstanding of how evolution works. Evolution is a branching process, where one species can give rise to new species over time through a process called speciation. Speciation occurs when populations of a species become genetically isolated from each other and undergo enough genetic changes over time that they can no longer interbreed to produce fertile offspring.
In the case of humans, it is essential to understand that Homo sapiens, the modern human species, is the only surviving species of the genus Homo. However, that hasn't always been the case. In the past, several other species of the genus Homo did exist, such as Homo neanderthalensis (Neanderthals), Homo erectus, Homo habilis, etc. Over time, these other hominin species went extinct, leaving Homo sapiens as the only surviving representative of the genus.
The reason why multiple species of humans don't exist today is a result of the natural processes of evolution, including genetic drift, environmental pressures, competition, and migration patterns. As different hominin species encountered changing environments and faced various challenges, some were more successful in adapting and surviving, while others were not. In this way, the process of evolution led to the dominance of Homo sapiens as the sole surviving species of our evolutionary lineage.
So, the absence of multiple species of humans today is not evidence against evolution; rather, it is an expected outcome of the evolutionary process and the history of our species' interactions with other hominin groups in the past.