Hey, thanks! I've only ever seen them down in South America... don't know what the deal is on that. This was a little field at about 16K and they were melting so quickly that I bumped a few and knicked them over....
My guess is that the snow starts to melt, with debris settling into the low spots. The debris then absorbs more warmth from the sun and the low spots melt even faster, leaving pinnacles behind.
On Cerro Bonete, on the descent after the summit, I wanted to hike over to a little sub-peak to the east. There was a narrow but tall penitente field in the way, and the fins were running perpendicular to my desired direction. They were so tall (about 2m) that I could not really step over them, and they prevented me from crossing!
About them being so common in only this part of the world, I suspect it is due to the climate (warmth and dryness) of this part of the Andes, which would facilitate melting, evaporation, and sublimation. I also could not help but notice the complete lack of trees at ANY elevation in this part of the Andes. This says strange things about the climate here, maybe that it is very dry, at least in parts of the year.
And to answer the questions, penitente are formed in the same way as sun cups; the heat of the sun melts some of the snow, and the water trickles down to a low point. Eventually, the water melts out cups, which then refreeze. Repeated, intense cycles of this are what form penitente.