I am not a sprinkler man but I would assume that the center force thrust would be different with a larger slope and effect the water distribution hence the limit of 2 in 12 slope.
Other places in NFPA 13 where slope is mentioned....
8.4.3(4) Extended coverage upright and pendent sprinklers installed under smooth, flat ceilings that have slopes not exceeding a pitch of 1 in 3 (a rise of 4 units in a run of 12 units, a roof slope of 33.3 percent), where specifically listed for such use.
See Figure 8.6.4.1.4 for roof or ceiling (wood) with slope 4 in 12 or greater.
8.7.4.2.2 Sidewall sprinklers, where installed under a sloped ceiling with a slope exceeding 2 in 12, shall be located at the high point of the slope and positioned to discharge downward along the slope.
That leaves me to think that supplementary sideways mounted sprinklers are called for with slopes greater than 2 in 12.
Some reading material...
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