So anything lower than 5 HD only becomes spawn, right? Well, the reason only creatures 5 HD and up turn full vampire is that only the more strong willed creatures in the world are capable of making the full change and tapping into their true potential. Vampire Spawn and slaves, the sheep vampires made from sheep stock. If everyone was capable of being a full vampire, including level 1 commoners, it would water down the template as a hole. By being an exclusive club, the vampires come off stronger fluff-wise.
I'm sure there is also a crunchy reason, too. Perhaps energy drain and ability damage should only be encountered at an CR estimated by HD.