Weight and safety would be A answers.

Hutch may be too young to remember, but SP used and still had cupola cabooses, used primarily by the mid 60's only on "locals", until possibly as late as the "mid to late" 70's (IIRC?).

As cars and clearances went higher, with things like TOFC and tri-level auto racks, you couldn't see anyhow. Also, as clearances went "up", and braking got better and better, on newer cars, and engines with dynamic braking, you didn't need to be in the cupola, to "take signals" from the "swing brakemen "up high", to set pops, or tie down the train to recharge the train, stopped on a hill. Things like that were and are still in the rulebook, that thankfully you never had to deal with, due to the technological changes, by the "second" generation diesel era, when I worked.

Less injuries, just as good,(if not somewhat better, as even later cupola cabooses had "bay windows, on some roads( a view of the "train" (to spot things like "hot boxes" before the age of detectors), and less "dead" (non revenue) weight to pull up hills is why many roads started going to bay window cabooses. These roads did it for the old fashioned reason, bay window cabooses, saved money. Then with EOT technology, the RR's got rid of the caboose altogether, and saved even more "money"...... SP was just one of the earliest and most firmly committed to the concept.

Back then, every kid in America wanted to ride the caboose. Today, there isn't one in a thousand kids out there, that would even know what a caboose is....... Other than maybe as good old American slang for your ass.....