As land creatures, we haven't explored the underwater tech tree in much detail. If there are fishpeople with mega brains out there somewhere, they might have figured out some stuff that we haven't.
(On the other other hand, salt water is metal killer...)
I think this is the key. The modern world is an electrified world. But the other things you point out are in some cases advantages are also disadvantages. But I think the biggest thing is metal and electricity.
They are stuck behind bronze age. I just don't see any technology without having these things as prerequisites. Even electricity is generally impossible when you live in a big conductor.