Because I haven't played in 5 years and there will still be at least 3 posts below this one saying "Woah! Wulf! Hey everybody, it's Wulf! THE FUTURE FATHER OF MY CHILDREN HAS RETURNED!" Maybe not in those exact words, but it's in the subtext if you know how to read it.
I played Siege because it ****ing matters. When I was a newbie running around barefoot in the woods, taming up 5 timberwolves at a time to go attack the ORC fort, it mattered. They learned my name, and they laughed, and it mattered.
When I started doing spawns with C8V, I got good at it, and the scrolls we got mattered. When we were attacked, I would defend them, and it mattered. When I disagreed with Uthrac and told him I'm out of the guild and to leave my spawn under penalty of death, it mattered.
I don't think C|C ever had more than 5 members at a time, but we mattered. I could run into any fight, any spawn, anywhere and people would either flee, sure in the knowledge that the embodiment of death itself had appeared before them, or they would rally their forces and come down upon me with the blazing fury of a thousand flaming suns. Every body that fell before me and every single second that I could withstand the unwithstandable odds, they mattered. I mattered to every dead newbie that I would resurrect and outfit in equipment 100x better than I looted off them, and I mattered to every gank squad that thought they'd found an easy target alone.
When I cancelled my account and let my house fall, it mattered.
Don't ask why you should play on Siege. Ask why you should play on any other Shard that just doesn't matter.