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 Perthsire (207 informants in total when those areas still had local Gaelic speakers) actually did record several items with cn- including cnoc. And there is not a single incident of this being anything but /kr/, same for the other cn- items cnò, cnap, cnàimh and cnuimh.

There is one variation for one of the three St Kilda speakers, informant 15, who has /kl/ in cnuimheag, but the other two St Kilda speakers have /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 it indeed once was /kn/ but that this has moved towards /kr/ for a very long time.

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]	+	-	+	+	+	-	-	-	+	-	-	-	-