Because if you don't use FS when the debuff is still at 18 seconds, then your priority may cause you not to be able to cast a buffed FS again until after the original debuff falls off, causing you to lose at least one tic of the DoT, which is more damage than the 1k you got from casting ES.
It's not guaranteed that the DoT will fall off in that time, but there is a risk. If you think you can judge whether or not you'll be able to use FS again before the DoT falls off, then by all means go for it, many enhancement shaman use that method. I only have the less efficient method listed in the guide because it's not a huge DPS difference either way and it can be confusing to a new enhancement shaman if they have to make snap judgments about their FS casts.