For editors of magazines, webzines and small-press anthologies, the key factor is once again time, time, time. The number of submissions still vastly outweighs the number of possible acceptances, and an editor working for love alone is likely to be also holding down a job to pay the rent. (Also, often, to pay some of the publishing costs.)
Nonetheless, you can expect someone to read your story eventually, even submitting out of the blue, without reputation or recommendation. For an unknown author, that’s the first hurdle cleared.
Not that a magazine, webzine or small-press anthology can afford to be without name authors. Names are an essential selling point, and editors will often approach name authors to request or commission a story. (Which later becomes a problem if the story turns out to be low-grade, bottom-drawer stuff …)
However, a few names are enough, and it’s not necessary for every author to be a selling point. There’s still room for relative unknowns to fit in.
For most magazine, webzine and anthology editors, as for most book publishers, the prospect of discovering new talent is one of the main reasons for taking on the task. The same obstacles stand in the way too: the brain-numbing effect of reading MS after MS after MS, the difficulty of bringing full and fresh attention to every story. Catching an editor in the wrong mood at the wrong moment can nuke your chances.
There are other reasons for rejection that have nothing to do with you. Every magazine, webzine or anthology has its own special slant and preferences. Some small-press outlets exist to push the editors’ vision of what SF, fantasy or horror ought to be. The wise author buys/downloads a copy and reads before submitting. Submission guidelines never paint the full picture.
Vision or no vision, American magazines, webzines and anthologies tend to focus on a niche segment of the market. The readers of, say, Realms of Fantasy or New Genre expect a specific type of story. Australian outlets are usually more eclectic, ranging across many forms of SF, fantasy and horror in a single volume.
Another factor applies to all outlets everywhere: a tendency to favour a known circle of writers and acquaintances. Not name authors but known authors. The smaller the press, the more this is likely to happen. But don’t complain too bitterly—these people are working for love, remember, not as paid professionals. The playing field may not be level, but at least you’re on it, you’re not excluded. You just have to play harder.
Finally, there are factors similar to those that shape a publisher’s list in book publishing. Your story may be rejected because the magazine, webzine or anthology already has a story on a similar theme; or because your story would tilt the overall balance too much towards magic realism or hard SF or whatever (the other side of eclecticism is a need for variety); or your story may be long at 6,000 words, when the magazine or anthology already has several long stories and is looking for shorter pieces. (For most webzines, 6,000 words will be too long in the first place.)
The moral is, don’t take every rejection as a reflection on the quality of your writing. Luck plays a part. Hang in and wait for the luck to swing your way.