I'm guessing that that 1939 wasn't actually circulating all those years -- it looks like it was on vacation for most of that time, or at least a nice cushy desk job. But it looks like there's at least one ding on the reverse that would've lit the fuse on a Zincoln -- the coin on the right may well have started its decay with a ding smaller than that.
I think the '39 barely has 15 years of circulation time on it. The 199x could have as little as 6 months.
What's really sad is the US Mint strikes the Lincoln cents they put in proof sets with the same crap metal. You would think with how much they charge for proof set they could use the original composition. I wonder if anyone has had they cents start to corrode in their proof sets.