One factor may be that they had a couple fatal accident at an early stage which resulted in big lawsuits due to if I remember correctly a design flaw.
It looks like only 23 aircrafts are still operational, and 13!! has been written off
Now, having 1/3 of your helicopters crash for any kind of reason would not help boost your sales.
As for the incredible climb rate, well you don't drain XX% of your power to drive the tail rotor for a start, then if you just put a big enough engine and a gearbox that can take it, then you get unbelievable numbers.
Quite a good web-page about the machine below..
Kaman K-Max Productionlist by Markus Herzig