Because she was old and barren (so she could not bear children), and her husband Ibrahim pbuh was also old. But the angels told her what when Allah Wills, then the matter is done.
(And his wife was standing by, so she laughed,1 then We gave her good news of ˹the birth of˺ Isaac, and, after him, Jacob. They responded, “Are you astonished by Allah’s decree? May Allah’s mercy and blessings be upon you, O people of this house. Indeed, He is Praiseworthy, All-Glorious.” They responded, “Are you astonished by Allah’s decree? May Allah’s mercy and blessings be upon you, O people of this house. Indeed, He is Praiseworthy, All-Glorious.”) Hud 11: 71-73
(˹They did not eat,˺ so he grew fearful of them.1 They reassured ˹him˺, “Do not be afraid,” and gave him good news of a knowledgeable son.2Then his wife came forward with a cry, clasping her forehead ˹in astonishment˺, exclaiming, “˹A baby from˺ a barren, old woman!” They replied, “Such has your Lord decreed. He is truly the All-Wise, All-Knowing.”) Ad-Dhariat 51: 28- 30
لأنها كانت كبيرة في السن و زوجها إبراهيم مثل ذلك.
(وَامْرَأَتُهُ قَائِمَةٌ فَضَحِكَتْ فَبَشَّرْنَاهَا بِإِسْحَاقَ وَمِن وَرَاءِ إِسْحَاقَ يَعْقُوبَ .قَالَتْ يَا وَيْلَتَىٰ أَأَلِدُ وَأَنَا عَجُوزٌ وَهَـٰذَا بَعْلِي شَيْخًا ۖ إِنَّ هَـٰذَا لَشَيْءٌ عَجِيبٌ ﴿٧٢﴾ قَالُوا أَتَعْجَبِينَ مِنْ أَمْرِ اللَّـهِ ۖ رَحْمَتُ اللَّـهِ وَبَرَكَاتُهُ عَلَيْكُمْ أَهْلَ الْبَيْتِ ۚ إِنَّهُ حَمِيدٌ مَّجِيدٌ ﴿٧٣ (هود 11: 71- 73
( فَأَوْجَسَ مِنْهُمْ خِيفَةً ۖ قَالُوا لَا تَخَفْ ۖ وَبَشَّرُوهُ بِغُلَامٍ عَلِيمٍ ﴿٢٨﴾ فَأَقْبَلَتِ امْرَأَتُهُ فِي صَرَّةٍ فَصَكَّتْ وَجْهَهَا وَقَالَتْ عَجُوزٌ عَقِيمٌ ﴿٢٩﴾ قَالُوا كَذَٰلِكِ قَالَ رَبُّكِ ۖ إِنَّهُ هُوَ الْحَكِيمُ الْعَلِيمُ ﴿٣٠﴾1)
الذاريات 51: 28 -30