Original: Despite its feathers, most palaeontologists do not consider Sinosauropteryx to be a bird. Phylogenetically, it is only distantly related to the clade Aves, usually defined as Archaeopteryx plus modern birds.
Simplified: Most paleontologists do not consider Sinosauropteryx to be a bird, because phylogenetically, it lies far from the clade Aves, usually defined as Archaeopteryx plus modern birds.