Ugos Matza

One of the Kellolim (rules) in Rashi is, he explains  the word when  it appears the first time

in the Torah or Gemoro.
12- 39 Vayofu es habotzeik – ugos matzohs . Rashi explains  Botzeik Shelo Hichmitz
Korui  Matzoh (unleavened dough is called Matzoh)
Why did Rashi wait until pasuk # 39 to explain what a matzah is?
The word matzaha appears in  Breishis 19 – 3  umatzhos ofoh vayocheilu.
It also appears in this same parsha three times before the pasuk of Ugos matzhos. 
1)Shmos 12- 17    ushmartem es hamatzhos --   2) 12 -18    boerev tochlu matzhos –
3) 12- 20 b’chol moshvoseichem tochlu matzhos.
Why did Rashi not explain what a matzah is in Bereishis or at least the first time in this
 parsha?

Answer:

 Yoma 75a :   By the Mon (Manna) it is written, Vetochanu , Ugos, and Lechem.

Lechem (bread-finished product) was for the Tzadikim ,  Vetochanu (needed grinding)

for Reshoim and  Ugos was for the beinonim.

Rashi explains the word Ugos “mashma kodem afiyah”  - before it is baked. The

beinonim didn’t need to grind the Mon but they still had to bake it.

It is not necessary for Rashi to explain what a baked matzah is. Every child knows

that an unleavened baked bread is called a matzah.

In pasuk 12 – 39 it says Ugos Matzohs , Rashi is explaining that not only

is an unleavened baked bread  called Matzah , an unleavened  dough

is also called  Matzah.