Because scientific evidence shows things go in that direction.
Scripture (kinds, baraminology) claims things go in a different direction, contrary to scientific evidence.
If you are doing science, you follow the first. If you are pushing religion you may follow the second (although that is a minority opinion even there).
From the article:
While nobody disagrees that there has been a general trend towards complexity - humans are indisputably more complicated than amoebas - recent findings suggest that some of our very early ancestors were far more sophisticated than we have given them credit for. If so, then much of that precocious complexity has been lost by subsequent generations as they evolved into new species. "The whole concept of a gradualist tree, with one thing branching off after another and the last to branch off, the vertebrates, being the most complex, is wrong," says Detlev Arendt, an evolutionary and developmental biologist at the European Molecular Biology Laboratory in Heidelberg, Germany.
The idea of loss in evolution is not new. We know that snakes lost their legs, as did whales, and that our own ancestors lost body hair. However, the latest evidence suggests that the extent of loss might have been seriously underestimated. Some evolutionary biologists now suggest that loss - at every level, from genes and types of cells to whole anatomical features and life stages - is the key to understanding evolution and the relatedness of living things. Proponents of this idea argue that classical phylogeny has been built on rotten foundations, and tinkering with it will not put it right. Instead, they say, we need to rethink the process of evolution itself.