为什么it is a good luck 去掉a

为什么it is a good luck 去掉a

luck是不可数名词，所以不需要用“a”
不可数名词前一般不需要加“a”或“the”等前缀

luck的解释，（当做名词时的意思）柯林斯英汉双解大词典 
luck /lʌk/  CET4 TEM4 ( lucking, lucked, lucks )
1. 
N-UNCOUNT Luck or good luck is success or good things that happen to you, that do not come from your own abilities or efforts. 运气
例：
I knew I needed a bit of luck to win.
我知道我得靠点运气才能取胜。
例：
The Sri Lankans have been having no luck with the weather.
斯里兰卡人一直没运气碰上好天。
2. 
N-UNCOUNT Bad luck is lack of success or bad things that happen to you, that have not been caused by yourself or other people. 霉运
例：
I had a lot of bad luck during the first half of this season.
我在这一季节的前半季遭了很多霉运。