why did daisy and tom escape

It's as simple as that he is her husband. Part of the deep sadness in the book is that Tom and Daisy "make sense" even though their marriage lacks a passionate love. Tom is not going to drop her - she's a good mate - and both her freedom and their reputation as a family could suffer from being too closely associated with the murder. So they leave town for a while

In Tom's own way he loves Daisy even though she fell for Gatsby. They are both obsessed with their wealth and station in life. I assume that literally "getting away with murder" would be something they quite like doing. Daisy married Tom for security and, in this instance; Tom supplies her with this,

"I called up Daisy half an hour after we found him, called her instinctively and without hesitation. But she and Tom had gone away early that afternoon, and taken baggage with them.

"Left no address?"

"No."

"Say when they'd be back?"

"No."

They left before Gatsby was declared dead because Tom wanted to show Gatsby that Daisy really loved him more than Gatsby.

And also, Daisy and Tom have a history of just leaving town unannounced.