1) It is a bit cheaper for manufacturers to use clockwise threads exclusively. Originally, French and Italian frame builders presumably did not want to pay royalties on the British patent for self-tightening BB cups.
2) This is not a fatal flaw, because the problem can be addressed with heavy torque and/or Loc-Tite.
3) I need to think this one through a bit further, but modern cartridge bottom brackets may have less tendency to self-loosen than their traditional cousins.
4) Several Italian manufacturers actually have switched over to English/ISO threading.
5) The French must have considered it a problem, because at least Peugeot and Motobecane switched over from French to the even less-common Swiss threading sometime between 1975 and 1980.
6) I have experienced fixed cup loosening on my first Bianchi and on one of my old Peugeots. Conversely, it was extremely difficult to remove the original English-threaded fixed cup from my Capo.
7) The two benefits of French or Italian threading are:
a) You can always remove the fixed cup.
b) If you install adjustable cups on both sides of the BB, you can fine-tune your chainline.

English and Swiss bottom brackets are engineered correctly, with a self-tightening, anticlockwise-threaded fixed cup. (I know this seems counter-intuitive, but consider pedals, which are also self-tightening, even though the right side is clockwise-threaded. The epicyclic action of the ball bearings generates a torque opposite the direction of rotation. If we used plain bushings instead of ball bearings, everything would be very intuitively obvious, and right-side pedals would have to be left-threaded.)

In a metric world, I am still amazed/amused that British BB threading won out over Swiss!