"We publish X because X works" is a suspicious statement. If publishers really knew in advance what would work, there wouldn't constantly be flops and surprise successes.
This boils down to argument from conformity: we publish X because we publish X.
If only books with fast beginings are published, then of course the majority of the reading public will be comprised of people who like fast beginning —since nothing is being published for those with different tastes.
I personally know people who hate books that start in media res, that don't explain anything about their characters, that give no context before several dozen pages in —because they can't care about the tension surrounding characters they know nothing about and thus haven't been allowed to emphatize with.
So of course, these people pretty much give up on modern novels and stop buying them.
If people want A or B, but you only give them A, then of course they only buy A —and then commercials conclude that people only want A, and so will never ever sell them any B.
That's the whole "video games are for boys" canard all over again.
Give publishers some credit. When a book flops, it's seldom because it started slow or started fast. it's because story wasn't well told, and/or the characters weren't empathetic, etc.
Publishers do their best to publish whatever the reading public says it wants, and they do a pretty darned good job. The publish many books with fast opening, many with slow openings, and many that fall in the middle.
It really isn't about slow openings versus fast openings, anyway, it's about
good,
engaging slow openings versus
bad, boring slow openings. Slow, fast, or anywhere between, the opening still needs story, character, good writing, and whatever it takes to make the book a page turned.
Publishers seldom reject slow, they reject boring, and it's readers who tell them which is which.
I don't believe for a second that anyone stops reading modern novels because all of them open to fast. If so, they just aren't looking around to see what's out there. Modern novels with slow openings are common.
I suppose I've read as many classic novels as anyone, and while many of them start slower than a lot of current novels, they don't stay slow for long, unless, just like now, the writer is telling a slow, non-action story. Most classic novels get to story and character pretty darned fast.
So do most modern novels. In media res doesn't mean you have to start with explosions and murders, it just means you start at a point in the story where character and events matter. It certainly doesn't mean a writer can't explain enough to make readers care about the characters before the whole world goes to hell.