qualitatively: Why are thioesters more reactive than regular esters?
http://guweb2.gonzaga.edu/faculty/cronk/CHEM440pub/thioesters.html

"thioester cannot make as much of a contribution because the required overlap between carbon's 2p orbital and the sulfur 3p orbital is ineffective. The large difference in energy between these two orbitals means they cannot "mix" as well as the 2p orbitals of oxygen and carbon."