Probably because it is an HID without instant strike. Since it is an HID repeatedly cycling it (turning it on and off) is bad for it (worse than with an incandescent bulb, I believe) and it does not achieve full brightness for 30+ seconds. 30 seconds does not sound like a terribly long amount of time to wait on paper, but for many people's uses when super-bright light is wanted, it is usually needed right away and only momentarily.
If you are just looking for a pocket stunner the
Microfire K2000R will do, but I see the Surefire M6 as a more versatile light (especially considering that it can take the longer running KL6 LED bezel and several different lamp assemblies.
Of course the most 'flashaholic' option is buying both...