NiMH cells are expensive compared to alkaline cells. A quick check on Amazon shows that, when buying a decent amount of AA cells at a time, alkalines go for about $0.28 each compared to $1.67 each for NiMH cells.[0][1] Add in a charger (and consider that most cheaper chargers are really crappy--look for NLee the Engineer's reviews[2]) and many don't consider the benefits to outweigh the initial capital costs. (Also, alkalines tend to operate at 1.5 V for a good portion of the voltage curve whereas NiMH cells are usually around 1.3 V. This can cause problems in some devices, although less so today than, say, ten years ago.)

I absolutely agree that NiMH cells are superior in almost every way, but it's not irrational to prefer disposable batteries.

[0] http://www.amazon.com/AmazonBasics-Performance-Alkaline-Batt...

[1] http://www.amazon.com/AmazonBasics-AA-Rechargeable-Batteries...

[2] e.g. http://www.amazon.com/review/RMWO7UZ9TAVRF/ref=cm_cr_rdp_per...