"The Interpreter's House still prints the odd story. So does "Tears in the Fence". And of course there's "London Magazine", "Stand", "Lighthouse" and "Under the Radar". I sometimes wonder how popular the poetry and fiction mix is. Will fiction writers subscribe to a magazine that only has a story or two per issue, if that? The fewer stories published in such magazines, the less likely it is that stories will be submitted. A few issues ago, "The Next Review" justified the lack of fiction on the grounds of submission quality, and the current (Spring/Summer 2015) issue of "New Walk" contains no fiction this time. From the editors' viewpoint it must be tempting to print poems rather than stories - contributors are likely to turn into subscribers if they aren't so already, and several poets' work can be squeezed into the space a story displaces.
My impression is that story-writers are less likely to appreciate poetry than poets appreciate stories. Consequently, story-writers are less likely to subscribe to mixed magazines. And more poets turn to story-writing than vice versa. Flash fiction (and especially microfiction) can bridge the divide. Interestingly, "The Next Review" sets a minimum fiction word-limit of 1500, blocking that route, whereas some other magazines don't label the texts published, leaving readers to classify short texts as either Flash, prose-poems or poems if they wish. But that flexibility risks putting side by side texts whose style differs only in their use of line-breaks and white-space, inviting comparisons that may be uncomfortable, particularly when the white-space is extravagant.