Because it's the only one that works for an interesting continuation. Green ending is basically "Galactic Peace" and Blue Ending is "Reapers force Galactic Peace".
While you could do a continuation that goes off Blue ending, it basically just boils down to "Shepard goes evil controlling the Reapers" so you're basically back to ME 1-3 of just fighting Reapers.
At least with Destroy, there's something different. The entire Galaxy is at square one as it is rebuilding. It'd basically be the wild west as there is no cohesive single government and everyone has gotten their shit wrecked. Hell wasn't a whole "wild west" aspect something they wanted with Andromeda?
And yeah, you could always "keep it vague" but because the blue ending specifically states that Shepard Reapers specifically help rebuild and police the galaxy, it'd be really weird to not have the Reapers show up anywhere and try to pass it off as either ending could have happened.
No matter what though, Shepard should remain dead.