Why Santa is krocking at your door

The short answer as to why Gaelic words that begin with cn- are pronounced [kr], for example, cnoc [krɔ̃xk], is that that's just the way it is.

That's actually not a trivial point to make because you'll find all sorts of folk pontificating about this issue - most of whom have no clue. The Survey of Scottish Gaelic Dialects which recorded people from Lewis to Barra, Strathy to Aran, Dunbeath to East Perthshire (in total, 207 informants when those areas still had local Gaelic speakers) actually recorded several items with cn-, including cnoc. There's not a single incident of this being anything but cnoc [krɔ̃xk] and /kr/ was reflected in the other cn- items cnò, cnap, cnàimh and cnuimh.

There is one variation for one of the three St Kilda speakers, informant 15, who had /kl/ in cnuimheag, but the other two St Kilda speakers had /kr/.

So, to begin with, we can be crystal clear that whatever the pronunciation was at some point in the days gone by, it is now always /kr/, usually with nasalisation on the nearby vowel.

It's also interesting to note that Manx and most of Ireland also have /kr/. Only Kerry Irish retains /kn/. That, along with clues from the way words with cn have been spelled and mis-spelled over the centuries, tells us that once it was /kn/ but that the movement towards /kr/ began a long time ago.

As to why... keeping the explanation simple, phonetically /r/ and /n/ are fairly close to each other. Don't be alarmed, here's a crazy looking linguistic schematic that will demonstrate just how close they are:

Sound	consonantal	syllabic	sonorant	coronal	anterior	high	low	back	nasal	lateral	continuant	tense	voice
[r]	+	-	+	+	+	-	-	-	-	-	+	-	-
[n]	+	-	+	+	+	-	-	-	+	-	-	-	-
[p]	+	-	-	-	+	-	-	-	-	-	-	-	-

You don't need to know what all these features. They're labels that linguists use to categorise the mechanics of a sound. What should be obvious is that except for two features, [r] and [n] are produced exactly the same way. Remember what I said about there usually being nasalisation near /kr/? That's the leftover from the nasal feature. So, if we take that into account, /knɔxg/ and [krɔ̃xk] are actually identical except for one single feature. The more features that overlap, the easier it is for sounds to jump.

In contrast, if you look at [p], it shares very few features with [r] or [n] so the chances of [knɔxg] turning into *[kpɔxg] are very slim indeed.

So, that explains why /kn/ moving to /kr/ is plausible and I'll leave the explanation at that. Sometimes such closely related sounds just flip in languages.