For a number of reasons:

 



1. In terms of the DNA found in the explosion, this
episode just established that Tyger, Tyger have resources at their fingertips
to call in a favour, no questions asked. It's conceivable that McAllister made
the call, as Red John or pretending to be someone else, asking for his DNA to
be switched in for the forensic analysis, and for the burnt crisp to be
identified as him. After all, if there truly are "hundreds" or
"thousands" of members of the Blake association, and this episode
showed that nobody could be trusted, I think that the positive DNA
analysis would be a very thin argument against his
survival.  

 



2. In terms of the explosion itself, he was
positioned (presumably still in the same position when the bomb went off)
alongside the other two suspects that survived, so unless he moved across the
room for some reason, the most you'd expect is that he would have been hurt by
shrapnel, but not burnt to a crisp like his supposed corpse.

 



3. General mannerisms during the confrontation. The
smile directed at Bertram and Smith as soon as his back was turned to PJ, the
fact that he was really the only of the three suspects with tattoos that shared
a brief conversation and moment of connection with PJ after revealing the
tattoo- looking him in the eye and saying he wasn't RJ seems like the kind of
thing RJ would consider a victory, if he managed to fool PJ once again. 

 



4. The actor's voice in other shows he's been in
has been shown to have the capacity to vary in pitch and sound quite similar to
the RJ voice.

 



5. For obvious reasons: the screen shot everyone
has of the 6x08 promo window break, and the nice Sherlock work shown in Theory #11952.

 



I would personally be quite happy for it to be
McAllister, not wanting to go over too much old ground with the whole Bertram
being too obvious now/Red Herring, and also that the showdown should always
have had that real twist to it, and this definitely would, much more so than
Bertram in my opinion. Although I do have to wonder at the people who put
together the promos for TV shows- unless it's a deliberate attempt at
misdirection by having McAllister in it, did they seriously not think that
people wouldn't be so eager to know the identity of RJ that they wouldn't comb
through the promo bit by bit?

 

Anyway, feel free to critique/criticise, I'm all
ears for other arguments.