I believe that it is just a holdover from the way GM always did mains in the past.
The same question can be asked as to why they used main shims after they stopped using babbitt bearings for the mains on engines much earlier that 1954.
It could be due to imprecise casting/machining capabilities of the time back then.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission