Why are phylogenetic trees revised?
They're always changing
Phylogenetic trees, much like the rest of Biology as a whole, are constantly changing. This just means when new information is available about a tree, our prior understanding of a certain phylogeny wasn't correct and it must be revised. The phylogenetic tree is split into many branches and we understand some of those branches better than others. Biology just happens to be one of those sciences that's always changing as we learn more and more, and the Phylogenetic tree of life is one of those areas that reflects this constant change.