Quite simply, because of the excellent lunches, all of which are prepared fresh every morning. The restaurant offers only one (very affordable), which changes daily. The food is universally excellent, and the service is very friendly, even when the one waitress is absolutely swamped when the place packs out at lunchtime (every day).
The price is around 15 euros for two delicious courses or 12 for just the plat, and the steak tartare here was probably the best I’ve had in Paris (too bad you can't order it whenever you like!) Everything is prepared from scratch, and is fresh and delicious.
Get in early if you want a table, especially if you want a table on the street in the sun.
Mon-Fr: 07:00-02:00
Sa: 16:00-02:00