Why can't the evolutionary relationships between certain species be explained thoroughly?

Their fossils are too large to study.

They have different physical characteristics.

A link is yet to be discovered.

The species are poorly related to each other.

Answer: The species are poorly related to each other.

Darwin gave information for evolution as "descent with modifications" it involves a process in which species change and gives rise to a new species over generations. He proposed that the evolutionary history of life forms a branching tree with many level as a new species evolve. All species can be traced back to a common ancestor. Each newly evolved species will have one or more characteristics in common as that of the old species.

In species with no common ancestor will not have any similar characteristic in common such species cannot be evolutionary linked and cannot be placed and linked in an phylogenetic tree.