Short answer

• In Jainism, the soul (jiva) is gender‑less. Becoming a Tirthankar depends only on very rare karmas (Tirthankar‑nāma‑karma), not on being male or female.
• In our current time‑cycle (Avasarpini), the 24 Tirthankars happened to be regarded as male in the Digambar tradition. The Shwetambar tradition holds that the 19th Tirthankar, Mallinath, was female (Mallī Devī). So it is not universally true that “all” were male.

A little more detail, very simply

• Jain Dharma teaches full spiritual equality of all souls. Gender is a temporary bodily form; liberation and supreme states arise from karma and effort.
• Digambar view: all 24 of this half‑cycle were male; women can reach the highest spiritual states but, for final moksha or Tirthankar‑hood, they take a subsequent male birth.
• Shwetambar view: women can attain moksha directly, and Mallinath Bhagwan was female in this era.
• Therefore, the reason you mostly hear “male” is sectarian history and the specific karmic outcomes of this time‑cycle—not a claim that women are spiritually lesser.

For simple, authentic explanations:

• On Mallinath as the female Tirthankar (Shwetambar view)
• On Mallinath’s life story and the gender difference noted by traditions
• On key Digambar–Shwetambar differences relevant here