In Jainism, the answer is different in Digambar and Shwetambar traditions.

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1. First, an important base point

• Soul (jiva) has no gender – it is neither male nor female.
• Male / female is only a temporary body, coming from karma.
• So spiritually, women and men are completely equal in capacity for purity and liberation (moksha).

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2. Digambar view – “Woman cannot be Tirthankar in this time‑cycle”

According to Digambar tradition:

• All 24 Tirthankars of the present half‑time‑cycle (avasarpini) are male.
• They say that the special Tirthankar‑nāma‑karma and the keval‑jnān (omniscience) needed for Tirthankar‑hood do not arise in a female body in this current time‑cycle.
• Therefore, a woman first takes birth as a man, and then can become:

- Kevali (omniscient), - Tirthankar, - and attain moksha.

So, in Digambar doctrine, it is not that the soul of a woman is lower; they say: for the final stage (Tirthankar / moksha) a male birth is required in this era.

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3. Shwetambar view – “A woman can be Tirthankar”

According to Shwetambar tradition:

• A woman can attain keval‑jnān and moksha in a female body.
• They also believe that the 19th Tirthankar, Mallinath Bhagwan (Mallī Devī), in this very time‑cycle, was female.
• So for Shwetambars, the statement “a woman cannot be Tirthankar” is not correct.

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4. Summary in simple words

• Jain philosophy itself:

– Soul has no gender. – Any soul can reach the highest state.

• Difference is only of sect‑belief about this time‑cycle:

- Digambar: In the present era, no woman becomes Tirthankar; she takes a male birth first for that final stage. - Shwetambar: A woman can be Tirthankar; Mallinath is accepted as a female Tirthankar.

So, “Why women cannot become Tirthankar?” Answer: According to Digambar texts of this era they cannot; according to Shwetambar texts they can. The soul itself is equally capable in both bodies.