Consequently, every magazine that focuses upon a particular interest is in direct competition with every other magazine focusing on that interest. Unfortunately, that is a situation that most editors absolutely hate. If you receive a rejection, you send the material on to Publication B, and wait again. If you receive another rejection, you try Publication C, and then Publication D, and so on. That sounds logical, and I always have a terrible time convincing writing students not to do it.
Editors hate simultaneous submissions.
While a few accept them, the vast majority still do not. One risk is the chance of offending an editor. If you withdraw your material, you risk alienating an editor who might otherwise have been a good market for your next story, and your next. A second risk is jumping too soon, and accepting the wrong offer. The sad fact is that you're stuck with the first contract you sign, no matter how good a subsequent offer may be.