From the editor's perspective, a closed anthology simplifies the workload immensely. Editors are first and foremost in the business of producing their markets. Easier workload means getting the book/magazine/what have you out the door with less effort expended. If you have ten slots, you invite twelve or fourteen writers and hope eight or nine of them make deadline. Then you go hunting for "savers" at the last minute as needed.
This also gives you a lot of advance control over the content. There are many writers, BNAs, midlisters and Young Turks alike, who can be relied on to turn in certain kinds of stories. You can make a fairly good effort a pre-balancing your anthology simply by being familiar with the work of authors you invite.
How those invitations get made is where a lot of the sensitivity comes into play. Who the editor has worked with before, whose work they like, who they can trust to make deadline, who can provide "cover pull" for the book — these all factor in to the process. Each of these factors also stands in the way of a newer writer, who hasn't earned trust, whose work may not be familiar, who doesn't have cover pull or an established relationship with an editor. It's not a matter of friendship — when I'm editing, I could stock a 1,000 page volume with stories by friends and still leave well more than half of them in the cold. I've rejected people I consider close friends, even when they've written on invitation, and I've bought stories from people I don't much care for, because they had written the right story for my project.
That's what a closed list is about — who the editor thinks can write the right story. It feels intensely unfair to writers who aren't on the closed list, even more so to writers who don't usually (or ever) get invited to closed lists. The point isn't fairness, or lack of fairness, it's getting the job done. If I (as an editor) can read 15 stories to buy 10, or I can read 500 stories to buy 10, and I can achieve roughly the same quality of product either way, why should I do all that extra work?
Sometimes I have done so, for example working with mme_publisher on her Polyphony anthology series. We were committed to open reading, and to making an extra effort to discover newer, and sometimes brand-new, writers. But that was an explicit commitment that translated to dozens and dozens and dozens of hours of extra reading. Every editor, every project by the same editor, has different budgets, priorities and timelines.
How do you get on closed lists? The same way you get anything else done in this field. By doing it. Sell to open markets. Anthologies, periodicals, Web site, whatever. Write more. Submit more. If your work is good, if it strikes a chord, you'll get noticed. Getting noticed eventually means you meet one or more of those editorial criteria I described above.
It's an imperfect system, but it does work.