I'm not surprised as it follows the trend of ejecting non-AEGIS integrated equipment wherever possible to seemingly simplify and speed up the design finalization/construction process for atleast the first few vessels. The RCN is desperate to get the River class under construction and in the water in order to start retiring the Halifax class, so it makes sense.
My guess is also that weight and space are potential concerns, as the Italian 5" gun is larger, heavier and has a substantial internal footprint. The 5 inch Mark 45 Mod 4 apparently has a weight of approximately 24,700kg~ in its largest form, while the OTO 127/64 LW has a weight of 33,000kg~ (+ or - 5%). The Italian gun also comes with a multi-drum automatic loading system which seems to take up more space than the BAE equivalent.
I've attached some images of both systems below for folks to compare.
View attachment 97411
View attachment 97412
View attachment 97413
View attachment 97414
Ultimately given the rumours and ongoing concerns with related Type 26 derivatives having eaten up substantial amounts of reserve weight for future upgrades just to implement the changes required, I am not surprised we are seeing some of the "gold plating" be removed and still good but obviously less capable design. The Italian gun has much more advanced/potent ammunition choices (more costly/complex though) for dealing with a variety of targets (drones), alongside a faster rate of fire and much deeper/versatile ready magazine system. This is definitely a tradeoff however if it can get back weight and space (potentially for more VLS?), it obviously must be important to be giving up important capability.