As mentioned by someone earlier, market research only takes you so far, though. As Henry Ford is alleged to have said, if he asked people what they wanted, they would have said "a faster horse." Or, as Wayne Gretzky said, you have to skate to where the puck will be. Apple has done this in the past. They killed their most popular iPod, the iPod mini, in order to deliver something no one was asking for, for instance.
But in those days of Apple Jobs was running the show, and actually believed he knew more about what the public wanted than the public did. And most of the time, he was right. But Apple is not the same company today.
No one loves their SE more than me, and I hope and pray we get a updated iPhone smaller than a 6/7/8 one day, but I think JPack is right on this one....us small phone lovers are a niche market these days, and Apple doesn’t seem to want to chase almost all niche markets (except for arguably the new Mac Pro).