Short answer

• It’s not “only 8 days” for all Jains. In the Shwetambar tradition, Paryushan is fixed as an 8‑day anushthan (observance) that culminates in Samvatsari (the annual Kshamapana/forgiveness day).
• In the Digambar tradition, the corresponding festival is Das‑Lakshan Parva, observed for 10 days.

Why 8 days in the Shwetambar tradition

• Scriptural-traditional observance: Shwetambar achar-granth and community maryada (discipline) prescribe an eight‑day period during Chaturmas for intensified sadhana—daily Pratikraman, tapas (austerity/fasts), svadhyaya (study, including Kalpasutra pravachan), and Kshamapana on the 8th day. This is why the span is set at eight lunar tithis, ending with Samvatsari.
• Practical focus: These eight days create a concentrated time-window for householders to step up vows, restraint, and repentance while sadhus/sadhvis are settled in one place during the rains. There is no single historical “event” fixing the number; it is a long‑standing religious convention within the Shwetambar parampara.
• Note on meanings: Some Shwetambar teachings also explain the eight days as a structured journey of purification that brings one to universal forgiveness on the 8th day (Samvatsari). The Digambar path emphasizes ten supreme virtues across ten days (Uttam Kshama to Uttam Brahmacharya).

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