Probably because the engineers realized that the front ends were kind of flimsy? Issue may have become more obvious with the 390-4V engines in 1967.
Progression:
Passenger side torque box included at the start of 1968 model year.
Reinforced shock towers on big block cars, starting in the Spring of 1968. Maybe coincided with the availability of the R code 428CJ engine in April? Earlier like March 1968?
Starting in 1969, extra bracing on the shock tower braces with 351 and larger engines.