Which organs do cats not eat on their prey?
That looks like some sort of rodent.  The rodent stomach, while not as acidic as the human one, is still strongly acidic: 


  The mouse stomach pH was 3.0 (fed) and 4.0 (fasted), and the corresponding values in the rat were 3.2 (fed) and 3.9 (fasted).


From this article

Since it's clear from the picture that the organism had just eaten (the stomach had not yet emptied), the pH would be on the lower end of that spectrum.

In humans, acidic foods often taste sour, so it may be that the cat ran into a terrible taste at the base of the esophagus that caused her to stop there, but I don't know whether the taste buds on the cat are comparable to those in a human.