Short answer: In Jainism, it isn’t about “India” versus “outside India.” It’s about where and when Tirthankars can be born according to Jain time‑cycles and cosmic geography.

• Jain cosmos: Our world is part of Jambudvipa, which has specific regions (kshetras). Tirthankars take birth only in karmabhumi regions—places where people experience both happiness and suffering, can follow vows, and spiritual effort is meaningful.
• In Jambudvipa, the Bharata Kshetra and the Airavata Kshetra are karmabhumis. During each half-cycle of time, 24 Tirthankars arise in Bharata and 24 in Airavata.
• We are currently in such a half‑cycle, and our human habitat corresponds to Bharata Kshetra, which broadly aligns with the Indian subcontinent. Therefore, all 24 Tirthankars of our present half‑cycle manifested in places that today we call “India” (ancient Ayodhya, Varanasi, Mithila, Saurashtra, etc.).
• This does not mean Tirthankars are limited to “India” forever. Jain texts also describe current living Tirthankars in Mahavideha Kshetra (like Lord Simandhar Swami), and in other eras/regions Tirthankars arise there as well. It depends on the region’s spiritual conditions and the phase of time, not modern political borders.

So, the reason is scriptural: the karmabhumi conditions in Bharata Kshetra during our current time‑cycle, not a restriction to a particular modern country.