Flexibility of long-chain aliphatic compounds
PubChem 3D has limits on the number of 'rotatable bonds'

I recently answered a related question about PubChem 3D:


  Beyond the limits of MMFF94 and MMFF94s themselves, PubChem3D had several limits indicated in the accompanying manuscript: Bolton et. al. "PubChem3D: a new resource for scientists" J Cheminf. (2011) v. 3, art. 32)
  
  
  Not too large (with ≤ 50 non-hydrogen atoms).
  Not too flexible (with ≤ 15 rotatable bonds).
  Consists of only supported elements (H, C, N, O, F, Si, P, S, Cl, Br, and I).
  Has only a single covalent unit (i.e., not a salt or a mixture).
  Contains only atom types recognized by the MMFF94s force field.
  Has fewer than six undefined atom or bond stereo centers.
  


Your molecule falls into the "more than 15 rotatable bonds" category. I count about 18 $\ce{sp^3}-\ce{sp^3}$ single bonds.

Basically, the problem is that the search space for conformers with that many rotatable bonds is too big. In principal, it's $\approx3^n$ where $n$ is the number of rotatable bonds.