The reason is that Satī’s father, Dakṣa, used to rebuke Lord Śiva in spite of Śiva’s faultlessness. Consequently, before attaining a mature age, Satī gave up her body by dint of yogic mystic power.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पितर्यप्रतिरूपे स्वे भवायानागसे रुषा । अप्रौढैवात्मनाऽऽत्मानमजहाद्योगसंयुता
pitaryapratirūpe sve bhavāyānāgase ruṣā | aprauḍhaivātmanā''tmānamajahādyogasaṃyutā
SB 4.1.66