Short answer: Because a Tirthankar is completely vitaraga (free from attachment and aversion) and observes absolute aparigraha (non‑possession). Ornaments and gemstones are possessions that create attachment, pride, and himsa; therefore, they are renounced.

A simple, authentic explanation:

• Aparigraha: A Tirthankar has given up every kind of possession. Jewelry, gems, or ornaments are forms of parigraha and are incompatible with the Mahavrata of non‑possession.
• Ahimsa: Gems involve harm in obtaining and preserving them. A Tirthankar avoids even subtle causes of himsa.
• Vitaragata: Their inner purity and keval‑jnana are the true “splendor.” They need no external adornment; such adornments only foster attachment in others.
• Samata and simplicity: Ornaments mark status and distinction. A Tirthankar embodies perfect equality and simplicity, so external displays are absent.

Tradition notes:

• Digambar: Tirthankars are depicted without clothes or ornaments, signifying total renunciation.
• Shwetambar: Tirthankars renounce all ornaments at diksha. Temple murtis may sometimes be decorated by devotees for bhakti, but this is symbolic; it does not mean the Tirthankars themselves wore gemstones.

For basics on vows and aparigraha, see:

• Mahavratas (including Aparigraha)
• Aparigraha and its meaning in practice