C NMR spectroscopy is much less sensitive (ca. by 4 orders of magnitude) to carbon than H NMR spectroscopy is to hydrogen, because of the lower abundance (1.1%) of C  compared to H (>99%), and because of a lower(0.702 vs. 2.8) nuclear magnetic moment. Stated equivalently, the gyromagnetic ratio (6.728284 10 rad T s) is only 1/4th that of H.
On the other hand, the sensitivity of C NMR spectroscopy benefits to some extent  from nuclear Overhauser effect, which enhances signal intensity for non-quaternary C atoms.