I think the story has a typo. LEO carried with 37 rounds, not 47. He carried two spare mags, so a Glock 21 w/2 spare mags is 40 rounds. He shot 33 rounds leaving four unfired. That equals 37. I figure he downloaded each mag to 12, so 3x12 + 1 = 37.
Since you can carry a lot more 9mm ammo than .45ACP ammo, I figure that's why he changed. 9mm is also easier to shoot, and has less recoil so faster target re-acquisition.
145 rounds of .45ACP would be very bulky and heavy. I imagine 145 rounds of 9mm is bad enough.