I think it's because radial tires naturally track straighter than bias plies. Most modern radial specs require almost zero toe or maybe 1/32".

Bias ply tend to follow imperfections in the road and want to push outward while driving so toe-in helps keep the car in a straight line by countering this.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)