Why can't I find any Skylanders of the Life element?

Only 2 of the 4 Life element characters have been released so far: Stealth Elf and Stump Smash. The other 2 (Zook and Camo) are coming out in a later wave. Since I'm pretty sure they made equal numbers of every character, that would mean that there's half as many Life characters available than, say, the Tech element, that has had all 4 characters released. So, anyone who wants to 'complete' their basic 8 elements to unlock all the elemental gates would need to track down either Stealth Elf or Stump Smash, thus it makes sense that the Life characters are harder to find (higher demand relative to elements with 3 or 4 characters available now).

So I guess the answer is 'yes', they are holding them back. The release dates may get moved up based on the demand, but it was always in the plan to release new figures in waves.