@mansr There is no canon explanation. It seems to be tradition for whatever reason, but e.g. Saavik and Valeris had parents that broke tradition.
Fan theories include:
- Honoring T'Pau (Nimoy's speculation, but as of ENT, T'Pol was named before T'Pau was influential)
- "T' " means "of", and maybe it was originally given when you marry and become of your husband
- The word "t'sai" is "lady", maybe it leaked into names
- Maybe those few not named T'Something were conceived outside pon farr
(information stolen from T'Kay, Vulcartist)