The 60-250 is an awesome lens, and I think that those who make the commitment to buy it won't be selling it off anytime soon - no surprise you don't see many on sale used.
For some of your questions - it's about the most expensive Pentax lens on sale at the moment. So stores are going to be reluctant to stock something like that in any quantity, especially since so many Pentax owners aren't used to spending so much on lenses. All Pentax lenses tend to be low quantity sales items anyway and stores can't afford to carry expensive inventories.
As far as demand goes - here are my personal reasons for choosing to skip the DA*60-250:
First, I have owned the 50-135 since they first came out well before the 60-250 was released. So it wasn't a matter of buying either/or, it was a matter of would the 60-250 offer me enough more to buy it in addition to the 50-135.
Second, I took a look at a whole bunch of Pentax lenses when Pentax did their K7 Tour. I was looking for a high quality lens in the 200 mm range. I had previously looked at the Tamron 70-200 f2.8, found it at the edge of the weight I could manage for hand-holding (an absolute necessity for me). At the tour I tried the DA*200, DA*300, DA*60-250, DA*16-50, and DA 10-17. All of the * lenses are amazing! However, while I could manage both the DA*200 and DA*300, I found the DA*60-250 just too big and heavy for me to handle comfortably.
The third reason for choosing a combination of DA*50-135 and DA*200 is that both of them are f2.8, while the DA*60-250 is f4. For some people that extra speed is very important. That wasn't a big factor for me as I wanted that range mostly for outdoor stuff but it was fairly high on my list of priorities when I bought the 50-135 and I'm often very grateful I have it.
Finally, a last reason for some people is the fact that the DA*60-250 doesn't go out to 300 mm. The DA*300 is almost as much as the DA*60-250 and many people might not want to spend that much for both lenses. So I can see why this lens would be less popular with birders and wildlife photographers.
That K7 tour was quite expensive for me, and if I am any indication Pentax made money on it (I wish they'd come around again, though my hubby wouldn't agree!). I ended up buying the DA*200 and it's become one of my favorite lenses. While it wasn't high on my priorities, I love having f2.8.
Even if I had been able to handle the DA*60-250 comfortably, I still wouldn't have wanted to sell my DA*50-135. I can't imagine walking around West L.A. taking street shots with such a big lens, the 50-135 isn't exactly small (length 136 mm, weight 685 g) but it's more normal sized compared to the 60-250 (length 168 mm, weight 1230 g).
But there's no denying that the DA*60-250 is a brilliant lens, image quality is outstanding.