why increasing calcium concentration increase the force of contractility in case of cardiac muscles not in skeletal muscles?

 in skeletal muscles , a single action potential releases sufficient calcium to fully saturate the troponin sites that activate contraction.

but
read more in cardiac muscles ,the amount of calcium released from the sarcoplasmic reticulum is not usually sufficient to fully saturate all troponin sites,

so the number of active cross bridges and thus the strength of contraction can be increased if more calcium is released from the sarcoplasmic reticulum .