This is most simply put because, Apple doesn NOT make a 16GB model anymore. The new gen touches (summer 2009) only come in 32 and 62GB versions and are similar to the iphone 3gs in terms on speed and graphics etc. the 8GB version is similar to the iphone 3g in terms of speed and graphics etc. i have no idea why Apple dont stock a 16GB version anymore but take my advice and buy a 32GB model, you won't regret it!
Hope i helped, Liam AKA CincoTres