First all, short is good. I don't seem to have enough patience lately for longer works, and that's probably not a good comment on my quality of life these days.
But there are other, better reasons. With a short poem, it's easier to keep the whole thing in your head after you've read it, and continue to let the words ring after you've closed the book or left the computer. To point out some of the other reasons, this is one of my lasting favorites:
He who binds to himself a joy
Does the winged life destroy
But he who kisses the joy as it flies
Lives in eternity's sun rise
I find much within these four lines. I treasure its insight and vision. They invalidate conventional thought. We all told we must try, try, bear down, and really want it if we are to succeed. But the joy, the very reward of doing what we desire is crushed when we clutch it fiercely.
No matter what the discipline: poetry, sculpture, or painting, the works where the artist has been able to "kiss the joy" are the ones that endure. And there you have a second thing I prize in a poem, a memorable phrase or snippet that works both as words, and as a focus.
This poem is very regular and tight, flowing easily, right up to the end. But there, "eternity's sun rise" breaks this regularity. The tone and infection of "sun rise" gives a different kick at the end. It pushes very close to falling apart, but it doesn't. This is a poem that works its form instead of simply complying to it.
When I read a poem, I don't do so with the expectation that it needs to be a masterpiece to be worthy of my attention. In reading poetry, I'm willing to kiss the joy and savor whatever I find.