why the author use ”and, ” not ”, and” ?

It is correct. Usually, in a sentence, the phrase between two commars are additional information, which can be neglected with the sentence still making sense.
The Brown bubble has burst and, thanks to Labour's economic incompetence, nothing was put aside for a rainy day to help British families cope.

The phrase highlighted can be neglected, because it's between two commas.
Without the phrase, the sentense is like this:
The Brown bubble has burst and nothing was put aside for a rainy day to help British families cope.
If the commar is in front of 'and', the phrase in the middle is important and cannot be neglected.
Some other examples:
My neighbour, Susan, is happy.
'Susan; is additional example and can be neglected.

Madam 答得好好，好清楚呀！只是用英文答，其他回答者看不明白。

「thanks to Labour's economic incompetence」是附加的資料，如果唔要變成
「The Brown bubble has burst and nothing was put aside for a rainy day to help British families cope」也是完整的句子。

所以在附加資料的前和後加上「，」。
反過來說：意即兩個「，」中間的是附加資料。

如果不是中間加插了附加資料，and 的後面便沒有「，」。

2008-09-13 16:46:06 補充：
加少許補充：「and」的前和後，一般都沒有「，」

oh sh!t!!!我唔知!!
你係鬼妹嗎?