My guess (I'm not a physicist or engineer.) is momentum. Lighter slides have less momentum. That means they're easier to slow down with things like hammer springs and flat bottom FPB (see Bill, I was reading! ), so not as much frame battering, less reciprocating mass up high, but also less umph to close an a slightly out of spec round, less reliability to go into battery when dirty or without lube, etc.
Heavy slides, in other words, make reliability easier, I suspect. Thus, many companies who mass produce duty guns go with them, despite the fact that lighter slides may give some desirable characteristics as far as shooting performance.
By all means, if I have this wrong and you know it, correct me, but that's what I've gleaned.