A brief history:
In the beginning, there were death stalker prism cannons and firestorm tanks. They were two separate tanks with separate roles and it was good. And lo, Epic 40k did combine these into the fire prism, giving it both roles because GW was too miserly to make yet another vehicle for the eldar after having completely redone the line of models. This was April 1997 (rules) and October 1997 (models).
Immediately after this the plastic falcon kit for 40k was released (January 1998), and then a few months later the fire prism (April 1998). That was in the last few months of 40k 2nd edition (I still have the datafax from the white dwarf) and 40k 2nd edition didnt have flyer rules. Therefore, the stats in 40k made no mention of the AA ability of the fire prism.
3rd edition rolled around quickly (November 1998), and GW had this idea of simplifying the rules to make the game faster so players could actually finish a game in a couple hours. Soon enough the eldar codex was released (September 1999) and the fire prism rules were basically a simplified version of the 2nd ed stats (minus the secondary diffused fire mode). At that time FW had yet to create their flyer rules, so still no AA ability for the 40k fire prism.
When flyer rules were finallized (in Imperial Armour in 2000 and later in Chapter Approved in 2001), none of the existing vehicles in 40k were retconned into having AA abilities because that probably would have been too confusing for people. Instead, FW resurrected the firestorm (in May of 2004) to act as the eldar AA tank. Interestingly, this kit was developed in the middle of the time that stormwind was being play tested (most of 2004). And so, the firestorm got inserted into the stormwind list because it allowed GW to add an extra vehicle for very little effort.
Of course, many of the play tests for swordwind were done before the firestorm was even hinted at. At that time the eldar needed an AA tank, so that role fell to the fire prism because it was the only option and that was how it was used in Epic 40k. The firestorm was revealed after a large number of playtest games were logged with the fire prism as the sole AA tank. By that time the fire prism couldn't just be changed to match the 40k version, because that might upset the system and the release date was looming.
Finally, after release of swordwind, it became clear that all the play testers had been high or drunk, because none of the swordwind lists were balanced against the original rule book armies. Thus, the Eldar list was adjusted to remove spiritstones and change the pulse rules. At the time this was done, the opportunity was given to change the fire prism to better reflect the role it has/had in 40k for nearly 10 years - ie tank hunter without AA.
Thus the Fire Prism was changed from the swordwind (2005)/ Epic 40k (1997) stats to the more 40k 3rd (1998) through 6th (2012) -fiendly stats we have now.
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I seem to recall it turned up in the printed book without having been included up to that point - I.e. it was added on a whim by Jervis at the last minute. It proved totally OP of course, so was removed again in the errata. Likewise, Pulse was not changed for balance reasons but because it vastly encouraged sustained fire and made doubling a very poor choice. (We had this discussion on another thread recently).