I am not certain but from what I have seen in previous years it seems Nikon and Canon throw always the newest stuff on the market - may it be video or what ever else is fancy at the time. With Pentax usually it seems they wait and test to produce something that works well before they put it out.
And yes, the commercials too are to some part the reason for it. From all magazines I have read in the past couple of years I have seen Olympus, Nikon, Sony and Canon commercials but the big A4 pages of Pentax? Cannot remember a single one of them.
But then not putting that money into commercials also means the over all costs for a Pentax are lower than for a Nikon or Canon of the same quality...
And yes...for some strange reason Pentax is often not on display in the stores either. One of the big Scandinavian stores does have it on stock but they haven't got a single one on display. Makes one wonder if Canon and Nikon pay them for being put out on display too. Gets funny when you get in there and actually buy a Pentax, because when I bought my last one they told me what I great lens I had bought but that so few would but Pentax these days. Who wonders? They have classes for beginners on Canon and on Nikon, not on Pentax. Pentaxes aren't on display. And I seriously wonder if when a new customer who has never had a DSLR (or for that matter SLR) comes in and wants to purchase his/her first one...do they tell them about Pentax?
I remember that when I bought my old K20D I had made a list of all advantages / disadvantages compared to a similar priced Nikon (D60?) and Canon. The Canon was kicked out of consideration rather early. I knew I did not want to start Video as I have never been interested in it anyway. So the main differences between both were:
Pentax: anti shake in the body (yeah! Saves one quite a bit of money over time) but no video
Nikon: no anti shake in the body, but video.
The Nikon was if I remember correctly 100€ more expensive. Which made me wonder what they had forgotten to implement (except for the obvious anti shake) that they were able to price it that low despite a then fancy new technology included...
So my decision was made - I only had to hold it before purchase, as if it had not fit my hand I would have had to start the search from scratch again.
But then...while everybody else these days "Is Nikon" and by that is the same as his neighbor and the neighbor after that... we are a special bread and not like everybody else on the block ;-)
Maybe that's the reason why Pentax is the only camera on the market that still is delivered with a printed manual? There are so few of us that they cannot count on us asking next door