Because contrary to the popular opinion in tech forums, bigger phones are more popular with the general public. The iPhone Pro Max is more popular than the Pro and likewise with the Galaxy S Ultra compared to the rest of the Galaxy S models. Don't believe me? Check Counterpoint or any other market research study. Granted more manufacturers should make more phones that are iPhone 16/Galaxy S25 sized as the base iPhone ends up on top of those charts every single year but it seemed like Chinese OEMs didn't want to do that until they could fit a really large battery in there. They're doing that now but it seems like global availability is limited.
Always makes me laugh when small phone users blame the lack of popularity of the mini on the iPhone SE and completely ignore the huge difference in price. Price may not be considered important in enthusiast circles but it is probably the deciding factor for a vast majority of buyers. It's also funny how the regular sized iPhone still ends up being the best selling phone in the world despite the SE being in between both sizes. Apple tried the mini experiment for two years, I wonder what's the excuse for the 13 mini being a failure despite the SE not getting an update that year. Yeah it got one in 2022 but the mini still sold abysmally compared to the rest of the lineup if you look at Q1 results.
Additionally if the SE took sales away from the mini like it's fans say then shouldn't it have taken away sales from the base iPhone as well? Yet they never make that argument and insist on coming up with even more outlandish theories like saying a Pro version would be the best selling model globally when logically it would have garbage battery life to fit the Pro models cameras. We've already seen this with the iPhone 16e as it manages to fit a significantly larger battery compared to the iPhone 14e because it has a smaller camera that occupies less space.