Technically, a comet is a huge ball of rock, dirt, ice and frozen gases. In the series, Sozin's comet catches fire by entering the upper atmosphere, giving a boost to firebenders all around the world by simply having a lot of their element floating in the sky.
Wouldn't earth benders also be strengthened by the appearance of such a large rock? If nearby presence is enough for firebenders, why isn't it for earthbenders?
The same argument could go for waterbender, ice being a primary component of the comet and water vapor possibly accounting for nearly 80% of it's tail's gaseous volume (like in Halley's comet's case).
The case could even possibly be made for airbenders, as large quantity of gases would be released in the air by such a comet that actually entered the atmosphere. Also, it's entirely possible that the air movements caused by the passage of such a massive object actually create wind in the upper atmosphere.
Thus, my question would be "why are only firebenders affected by Sozin's comet?"
Preferably, an accepted answer would be based on known canon.
Thank you for your time, everyone!