Usage of で in this sentence


I have a sentence (jlptN4):

> その花屋はいつもきれいな花でいっぱいです。

Could someone please explain why it needs で here, not が?




> その花屋はいつもきれいな花 **で** いっぱいです。  
>  その花屋はいつもきれいな花 **が** いっぱいです。

Both sound good to me.

「XXでいっぱい(だ)」 means "(something is) full of XX" "(something is) filled with
XX".  
「XXがいっぱい(だ/ある)」 means _lit._ "XX is many" → "there are many XX".

So the former literally means:  
"The flower shop is always full of beautiful flowers."  
And the latter:  
"As for the flower shop, there're always a lot of beautiful flowers."

You can also say:

> その花屋にはいつもきれいな花がいっぱいです/あります。

_lit._ "In the flower shop, there are always a lot of beautiful flowers."

