one word: mediocre.
everything about this place demonstrates the word "mediocre". The food, the service, the price...holy mediocre.
On the plus side, it's better than "bad"! So, if you absolutely must satisfy a Vietnamese craving and can't go anywhere better, this place is not terrible.
That's the only possible reason I can think of as to why it can get so crowded. People just HAVE to have Vietnamese and there's nothing else really nearby.