Why is Eden capitalized in the poem Nothing Gold Can Stay?

In Robert Frost’s poem, the speaker does not capitalize Eden. The correct line from “Nothing Gold Can Stay” reads:

Nature’s first green is gold,

Her hardest hue to hold.

Her early leaf’s a flower;

But only so an hour.

Then leaf subsides to leaf.

So Eden sank to grief,

So dawn goes down to day.

Nothing gold can stay.