The biggest reasons in my mind are
-Weight
-Corrosion resistance
-Cost
I did not start carrying until I was 58yrs of age. I wanted something I could CC when in the office or out and about. I wanted reliability, concealability, ergonomic fit, low cost.
I asked my resident experts —my son, an ex-Army Reserve man; and my brother, a lifer in the Air Force, Army tanks, and Army Reserve (recon). My brother was old school from the get-go. 1911 .45 was the only wise choice. Only exception was a .45 revolver.
My son disagreed. He was carrying a G26, with a SP101 for back up. I tried them all. I found the G26 fit my hand and my needs. 1911 was just too big for CC. Revolver trigger pull was too long (I consistently fired low and left, pulled down by the wait for the for the shot to fire—you know, "wait for it . . . wait for it . . . when is that sucker gonna . . . damn! there it was! )
Went back to my brother (he was trucking at the time). He said 9mm was to weak, could not stop a BG. Gave me all the stories about BGs in the Philippines not being stopped by smaller calibers so the Army came up with the .45. Problem solved. Good reasoning, I thought. So I told him I was planning on carrying hollow points.
"Those are illegal!" he said.
"Not in Indiana. I checked it out. I plan to carry the G26 with hollow points. If I can't stop the BG with eleven (10+1) hollow point rounds I deserve what I get."
"Hollow points are legal in Indiana?! Well, go with the Glock, then."
Since my brother has been kicking my butt since we were kids (yes, he was older—but he was smaller and I thought I should be able to whip him. Learned my lesson. Never mess with big bro, especially if he is a lifer), I took his advice. Especially as it was supported by my brilliant civil (structural) engineer first-born son. I bought the Glock.
When the G26 proved a bit large for summer carry, I bought its smaller little brother (after all, little brothers are smarter )— Taurus 709 slim. Practice makes perfect, so I am still working on the accuracy; but I can hit my targets at 10-15 yds with either hand or weapon. Spread is wider than I like, but I am working on it.
Plastic works for me. The BG is down; does not matter if he was hit from plastic or steel. But if my only option was steel, I would probably be throwing rocks instead of lead.