Why is the skeletal muscle of a frog outside the body unable to contract, and the heart continues to beat for some more time?

1) The ability of the heart to contract outside the body (or, the ability of the heart to contract without external stimuli under the influence of impulses arising in itself) is called automatic. It has been established that in the heart muscle there are special cells in which excitations periodically occur, transmitted to the muscle walls of both atria, and then to the ventricles.
2) A distinctive feature of the heart muscle, in contrast to the skeletal muscle, is the presence of a large number of intercellular contacts (nexuses). These contacts are the site of the transition of excitation from one cell to another.