Certain highbrow critics (I won't name names, but I've reviewed the work of at least one of them here, and not that long ago either) would have us believe that this is because third-person omniscient is "passé."
Such remarks say more about them than they do about fiction today--their Modernist prejudices, not least their love of unreliable narrators and ambiguity for its own sake.
It also reveals another failing of this type of critic: their utter obliviousness to and disinterest in the more practical aspects of the writing life--which seems to supply the real reasons why we are getting so much first person writing, two in particular:
1. Given the preference for "dramatic" rather than "epic" storytelling (I'm using the Goethe-Schiller terminology here), and the emphasis on being "relatable" above all, they are understandably looking to foster an intimacy that will make the reader identify with the narrator. Not a new technique, just one stressed more than it used to be.
2. The old problem of telling and showing. "Show, don't tell" remains the pat advice of those who don't actually write to those who do--and is followed much, much less often than we are led to believe, for good and obvious reasons. One is that, as compared with showing, telling is much easier to read--which is enormously important in today's market. It is also much easier to write--which matters all the more given the expectations increasingly placed on writers (low pay rates, longer books, lots of hours devoted to publicity, all without their getting to quit the day job save in a few, fortunate cases).
The result is that unless one really regards Flaubert as their Penelope (and ready to spend five days agonizing over one page in the manner of the man who gave the world Madame Bovary), setting aside all concerns but pure literary craft, they will, in spite of the conventional wisdom (truly conventional but never wise) serve up much more "tell" than "show."
But in fairness there's often a certain sleight-of-hand involved, and that's exactly what the first person point-of-view permits. Because of the pretense that we are in the narrator's head, directly listening to their voice, their telling looks a bit like showing--and most of those flogging the old "Show, don't tell" saw let them off the hook.
