If my faded memory is right,, I recall reading somewhere that WHEN the change was made,, part of it was due to the machine plates that stamped the frame halves were worn, and they were having to replace them.
But that doesn't answer the "why" the changes were made, because they could have just duplicated the A-54.
As noted above, the magazine button, a person holds the magazine in his right hand, to depress the button as he's loading. Feeding bullets with his left hand. The A-54 type,, makes this more awkward. And as Dan mentions, the engineers may have been looking at changes for the bolt hold open etc long before production happened.
Remember,, often,, perceived problems, (complaints) by people often drive changes in an item.
Look at the grip frames on the SA's. From XR3 to XR3_RED, to the Dragoon. ALL of those changes came about because someone convinced Bill that things could be "better."
And if you study all 4 models of the MK series .22's,, most of us prefer the MKII over all the others.
The comparison of the Browning 1911,, it too had mods early on. From the 1911 to the 1911-A1. And then look at the current crop of 1911's. Most folks prefer a version of the A1 with other mods over the original design.
Evolution due to customer input.
Just my humble opinion.