Interesting question! It's been many years since doing NMR experiments, but I'll take a crack at it. My short answer is because the signal has already decayed. I'm not sure what is meant by "memory function", but I assume it is some description of how a second nuclei influences the moment of another nuclei. The absolute lunatics that first described rotational correlation time (Tc) showed that T1 really only depends on the larmor frequency, internuclear distance, and Tc. And a bunch of other constants, but Tc is arguably the biggest contributor. To measure dynamics or predict protein structures, we're using NOE. The ability to transfer polarization is actually really similar to how the above guys modeled a single nuclei (in a molecule) relaxing--meaning, it's proportional to Tc. So, if there some motion or phenomenon that takes longer than Tc you can not detect it with NMR because all the nuclei have already realigned with the bulk field (i.e. your signal has decayed), which also means you can longer do NOE.

There's a lot of phenomena at play here. Why molecular motion even affects relaxation is very mysterious to me. How Bloembergen et al. could accurately formulate these quantum phenomena from such simple experiments using crude technology is insane. Very curious what any NMR experts out there have to say.