AngryManatee
Sexy Ham Manwich
Species are expected often to have fuzzy and imprecise boundaries because evolution is ongoing. Some species are in the process of forming; others are recently formed and still difficult to interpret. The complexities of biology add further complications. Many pairs of species remain distinct despite a small amount of hybridization between them. Some groups are asexual or frequently produce asexual strains, so how many species to split them into becomes problematical.
Creation, defining things as kinds that were created once and for all, implies that all species should be clearly demarcated and that there should be a clear and universal definition of kind or species. Since there is not, creationism, not evolutionary theory, has something to explain.
Different definitions of species serve different purposes. Species concepts are used both as taxonomic units, for identification and classification, and as theoretical concepts, for modeling and explaining. There is a great deal of overlap between the two purposes, but a definition that serves one is not necessarily the best for the other. Furthermore, there are practical considerations that call for different species criteria as well. Species definitions applied to fossils, for example, cannot be based on genetics or behavior because those traits do not fossilize.