The two aren't mutually exclusive. Those who wish to use Bluetooth headphones can happily use them, while those who prefer wired can continue to use them. There's no reason smartphone manufacturers shouldn't support both.
They do, you can get the little dongle and it has superior audio quality to most audiophile DACs on the market.
But that 3.5mm port takes up a lot of room that could be used for more battery, backup antennas for when the user's hand is covering one of them, vibration motors etc.
I'm pretty sure you could find justification even for why top models don't support microSD card expansion. The problem with this line of argument is that if they wanted to, they could support both without any issues. The real reason is money. It's more profitable to have Bluetooth only when you also make AirPods, and not include storage expansion when you sell built-in memory options at a 400% markup.
