Why some new tactical XML flashlights do not accept RCR123's?

I have been looking at some of the new Tactical Flashlights out there which use the new XM-L LED and use 1x18650 or 2x CR123's. A lot, if not all of them specifically note DO NOT USE RCR123's. Why is this? The lights I am talking about are the Fenix TK15, Eagletac T20C2 and P20C2. I have been using the Eagletac T10LC2 for the past 2 years and love it! I can use RCR123's. I can keep a set in my light while a charged set is waiting for the ones in use to cut out. When that happens I throw in the charged ones in the light ,and dead ones on the charger and repeat the process. I would like to keep this routine. I have 18650's but they are not flat top. Another thing the lights mentioned above require. It is uneconomical for me to buy CR123's. Will it really make a difference if I use RCR's instead? If someone could explain why it is so critical? I would really appreciate it, Thanks in advance.
Dan

(I researched the forum for any possible duplicates but nothing that specifically addresses this. If it is out there, i apologize I was never really good at research, I had a tendency to fail research projects in school. )

Put simply and not getting technical: primarily because of the LED driver input requirements, the majority of affordable XM-L drivers do not accept voltages higher then 6.0v, they are manufactured that way due to the fact that it would be most economical to run high current LED lights off a single 18650 cell. For the LED driver to accept a wider voltage input range, the manufacturing costs jumps considerably.

RCR123s cannot sustain the huge current draw of most modern high current LED lights. RCR123s are only safe for about 1.3A max current draw. IMR123s can sustain huge current draw up to 4.0A, but run times will suck. Most people will find a 10-20min run time on their IMR123 powered light to be really disappointing I reckon. Good quality 18650 cells usually provide 1-2 hr runtime before needing to be recharged.

Primary CR123s are safe up to 3.0-4.0A current draw, but to keep running costs down, it would be used as a last resort.

All the fun ones accept Li-ions

No complaints about the 490 lumen 1xAA in my pocket..

EDIT: Fusion m8 answers question in 2nd post above
The most common reason is that 2x [email protected] = 8.4v.... 2 x primaries@3v = 6v. The electronics that are in a lot of flashlights, including older models (not just the new XML models), cannot handle voltage above a certain level (6v in this case) and can result in malfunction or damage to the light. Some lights can use multiple RCR123 cells, you need to look at the voltage range. Heat can also be an issue, RCR123 cells aren't recommended for use in the 4sevens Quark 123 mini. Some small, high output lights need a lot of current and require cells with high discharge rates (IMR).

Another important but often overlooked reason: RCR123s have germs... XM-L lights do not like batteries with germs...:sick2: