There are two reasons I wouldn't use silk thread for making a kilt.

1) The inherent strength of silk thread. Thread that is too strong can actually cut fabric, fiber by fiber. Silk is a long staple thread, wool is a short staple fabric and 'soft' in comparison to silk.

2) The inherent fragility of silk. While silk can last for many hundred years, it does so with great care. Not so with wool. The wool in a kilt will last a lot longer than the silk thread used to sew it.