Past continuous v past simple query

[fififalolaboms]

Any help with this query would be greatly appreciated. My student is studying for FCE and has this gap fill to complete (from Use of English B2 for All Exams by E. Moutsou):

While Mary was having a bath yesterday evening, I was in the kitchen. I WAS COOKING her favourite dish because I wanted to surprise her. When she came into the kitchen she couldn't believe her eyes.

My student's question is why can she not use the past simple:

I COOKED her favourite dish because I wanted to surprise her.

To me this query doesn't seem to fall into the usual past continuous with past simple meanings:

1. an action in the past continuous which creates a time period with which other actions, in the past simple take place,

2. a point in past time which is specifically mentioned and the past continuous action crosses it, or

3. specifying a period in the past and the past continuous action fills it.

Is it something to do with the
a) the use of because in the sentence and/or
b) is the paragraph creating a narrative and past continuous makes it more exciting or maybe neither a nor b!

 

[MikeNewYork]

Both the past simple as past continuous would work. The difference is in the timing. With the past simple, the cooking has been completed. With the past continuous, the cooking was still in progress.