Why? Refresh rate is mostly important for gamers, and refresh rate for high resolution displays is much more difficult and expensive.
You might as well say Apple should just drop back to 1080P like all the competition and forget about retina.
I was talking about it from a competitors and advertising point of view, it looks bad when a supposedly premium product is inferior in specifications compared to cheaper products from other companies. But that's only looking at one "marketing bullet point".
It's true that higher refresh rates is much harder to do on higher resolution displays and requires faster GPUs, and I didn't think to do a 1:1 comparison between refresh rates + resolution with their competitors. There's also the increased power requirements that goes along with that, which for portable devices would require a bigger battery, resulting in a thicker and heavier product.
However, in the same price range as Apple, there are devices that have high resolution displays running at 120Hz and higher, so I don't see why Apple couldn't do the same. (edit: most competitors seem to have 2K displays)
Not that I care about refresh rates anyway, I'm over 50 and I've lived with both 30Hz and 60Hz all my life, I probably can't tell the difference between 60 and 90Hz, let alone 120 and 240Hz.
Which makes me question how they arrived at their choice, because 90Hz seems weird. Did Apple do polls internally and externally to test average Apple users? Did they test to see if most people could tell the difference between 60, 90, 120, 144 and 240Hz? If they did that and 90%+ of the people couldn't tell the difference between 90 and 120 Hz then of course it would make sense to go with 90Hz.
edit: I didn't even think about the bandwidth required for HDMI. A lot of the comments in this thread, my initial post included, must stop considering refresh rate by itself since it omits other requirements.