Probably because "the flu" is hundreds of different viruses and the entire world is not currently dumping massive amounts of resources behind developing them.

Then again, the rapid flu test only takes about 5 to 20 more minutes than this, and doesn't really cost much more than any routine diagnostic.

I'm sure the are different types of flu tests, but the last one I had at the doctor's office about 2 years ago was two swabs of my nose, which the doctor put into a test kit, and a few minutes' wait for the results to develop in the kit. No idea how much it cost, though.