She dived into the snow after the mouse, but the mouse escaped from the fox.
I saw the above in one of the videos about sentence variety.
Please explain the use of "but" in "but the mouse...
Why "and" doesn't make sense?

"But" is used to contrast.


The boy is handsome but not friendly.

The fox expects to catch the mouse. The fox did not catch the mouse. Thus, using 'but' contrasts the outcome vs the intention of the fox's action.