Usually, I'd just say one of the many reasons since the pros are just too long and most such ppl who ask such questions are not really looking for an answer :
1. Backwards compatibility to lenses (no kenna 'tua' by the whole mount change over time)
2. Fantastic SMC contrast and colors
3. Cheap, legendary and as good if not better optically old lenses
4. Lots of good old usable lenses to seek out and waste money on
5. Good small lenses (and not some 'tua leng kong' over sized lenses that dangle on my neck)
6. Well thought out camera bodies
7. Value for money cameras
8. SR on camera (don't need to pay for it every lens)
9. Pentax community
10. Weather sealed lenses on high and low end lenses. (other brands you pay top dollar as its on top lenses only)
11. Weather sealed body at a lower price
12. New optically as good lenses for lower price (except for the Canikon plastic fantastics of course)
13. I am not sheep (well I am sheep; the black sheep
)
14. I am an old Pentax user
15. Limited Primes; perhaps the ultimate balance between size, focal length, aperture speed, bokeh, sharpness
Frankly, I don't even bother what ppl think. All cameras can produce nice photos. Its the photographer that counts. Pentax just offers what I justify as preferable to my needs.