I think, among other things, it has to do with the size of the swap space relative to the speed of the swap device. IME high disk i/o combined with large swap space means swap never fills up and the OOM killer doesn't kick in. On systems with less RAM and swap, OOM conditions were hit much sooner, even with slower disks.

Default settings for dirty ratio and dirty background ratio exacerbate the issue: more data is held onto before it is written, and once the background ratio is hit, any application writing to disk will block.