Quite simple. We are Prime Azeroth, we are the only "trend" on the "rope" that really matters. We are the center piece of Time.
"In all known realms of the cosmos, time flows forward, ever forward. Chaotic energies in places like the Twisting Nether can affect how quickly it flows, but it only flows forward.
Once an event happens, it cannot be changed. These events and choices, made up of all creatures and forces in the cosmos, join together like a river, sharing the same reality. Different choices and different possibilities naturally spin off the river of time like small creeks and estuaries, ebbing and flowing for a while. If these shades of what could have been are left alone, they will eventually dissipate into nothing. If efforts are made to preserve them (or alter them), they can indeed remain in existence indefinitely. They can even be made to feed back into the main river—dead creatures can seemingly "live again," and the past (or future) may literally come back to haunt you. This is not a natural phenomenon, and the inhabitants of the main timeway will often find these experiences to be quite alarming.
But the only timeway that has a permanent effect on the cosmos is the main timeway. Creatures like the bronze dragons, who have command of temporal magic, can see all the countless tributaries of alternate universes and timelines, and they can even move back and forth along the stream to observe the past and the future.
If that main river is disrupted, it could spell doom and disaster. All life on Azeroth depends on time to flow ever forward. Without the surety that the sun will rise and set each day, the seasons would not pass, the cycle of life would become meaningless, and all living creatures would eventually die from being unable to sustain themselves. It is the most sacred mission of the bronze dragonflight to keep that from happening." - Chronicles Vol. 3