Obviously because she (ra) did not want to disturb the Prophet :saw: while he was praying. Thats why she preferred to leave quietly, without causing any distraction.
An explanation of this hadith:
Apparently, there was a discussion between Hazrat A’isha (ra), the wife of the Prophet :saw:, with some Muslims (who were not knowledgeable about religious maters) around what would nullify salat. She told them that she was knowledgeable about such rules. They remarked that when a woman, or a dog, or a donkey passed in front of a praying person, the latter’s prayer was nullified.
She corrected their wrong understanding with a sarcasm (of equating woman with dogs) and said that when the Prophet (S) prayed his tahajjud at her room, her bed (where she lay) was in front him. That is, those Muslims were wrong in their understanding of the rules of nullification of salat: salat is not nullified if a woman (mehrum) is in front of a praying person. Other ahadith that closely follow [in Bukhari] further clarify the matter (see below).
Volume 1, Book 9, Number 491:
Narrated 'Aisha:
The Prophet :saw: used to pray while I was sleeping across in his bed in front of him. Whenever he wanted to pray Witr, he would wake me up and I would pray Witr.
Volume 1, Book 9, Number 492:
Narrated 'Aisha (ra), the wife of the Prophet, “I used to sleep in front of Allah’s Apostle :saw: with my legs opposite his Qibla (facing him); and whenever he prostrated, he pushed my feet and I withdrew them and whenever he stood, I stretched them.” 'Aisha (ra) added, “In those days there were no lamps in the houses.”
Volume 1, Book 9, Number 494:
Narrated 'Aisha: (ra) (the wife of the Prophet) Allah’s Apostle :saw: used to get up at night and pray while I used to lie across between him and the Qibla on his family’s bed.
. Fine… as muslims and in the respect of ramadaan i will try to get as low as some people.