Just guessing, but my gut reaction is that steam probably has nothing to do with it. I mean how much steam can there possibly be in play here ? It's hard to imagine that the momentary clouds generated by draining pasta could possibly infiltrate an enclosure and the transformer itself enough to do damage. One alternate explanation that comes to mind is that the sink-side lights are turned on more of the time, or switched on and off more often, wearing them out sooner. Do you have them on a dimmer, and if so, is it a proper low-voltage dimmer? You could even chalk it up to chance; two consecutive failures isn't quite so unlikely as, say, winning the lottery.

It wouldn't surprise me if build quality had something to do with it. Not everything sold in the big-boxes is junk, but some of it certainly is.

A line-voltage system would have fewer components to fail, but you might find the choice of track fixtures rather limited.