I think it is more industry/equipment related. When working with power panels we usually refer to A phase-B phase-C phase. On some CNC equipment and VFDs you might see U,V,W and R,S,T depending in input/output. Three phase transformers are usually marked H1, H2, H3 and X1, X2, X3 for primary and secondary respectively (if there is an X0 it would be the neutral.) As Mike noted, L1,L2,L3 is common on industrial diagrams and line side of contactors-T1,T2,T3 on the motor side.
I am sure there is probably a NEMA or industry standard somewhere-seems to be too consistent to just be the random preference of the design engineer-though there are a few that seem determined to be creative!