Back in the late '60's I used to work for a small publishing company that published business magazines. We had a photography dept. that had two full time photographers. I was an editorial assistant and eventually was required to take pics to accompany my stories.
To do that I bought a new 1968 Pentax S1a and a hand held Sekonic light meter. My company told me to get either a Nikon or Pentax SLR. The S1a was cheapest, I got that, it was/is a good, rugged tool and I've been a Pentaxian since...but also have Leica RF, Mamiya MF, Olympus, Canon, Zeiss, etc.
Within the company (1960's) there were the two photographers and a couple of editorial types who took pics for publishing. We had a variety of cameras, mostly Pentax (Spotmatics for the most part), one Leica rangefinder, a Nikon F and one of the photographers had a Mamiya Press camera.. The publisher also had a collection of cameras that were medium format. He allowed the two photographers to use these when they were needed....ie; outside front cover for editorial and inside and back covers for advertising. The medium formats included a Linhof, an old Speed Graphic, a Rollei TLR and a Hassleblad.
At this time in the '60's what I recall newspaper press photographers around here, were using mostly Nikon F, Rollei TLR, some Mamiya TLR, Leica RF and the odd Pentax.
No Canons, but then this was just a bit before the Canon F1 was introduced by Canon. Canon made a concerted effort to get their pro cameras...starting with the F1 into the hands of pro photographers. At that time Nikon seemed to dominate the pro scene, when it came to 35mm bodies.
I left the publishing biz by 1970 and went into a completely different field of endeavour which I pursued till I retired a few years ago. So I lost touch with what was the pro press photographers first choice.
In around 2005 or so, I happened to bump into a long time press photographer and noticed he carried a Canon pro body. We shot the breeze for a few minutes and I said I was surprised to see him carrying a Canon, not a Nikon. He indicated that his newspaper (the big one around these here parts
) had switched over to Canon a number of years back.
From what I recall and I maybe in error (long time ago and I'm an old guy
he said reasons included deals on lenses from Canon and Canon were good at repair turn around with loaners if need be to pros. Nikon was ok in those aspects too, but his paper wanted to standardize their equipment for economic reasons. In other words if you have some Nikon shooters and Canon shooters on one paper...it gets expensive for the paper, if it's equipment pool has to buy both a Nikon mount and Canon mount 400mm F 2.8, etc. that will be used only occasionally. Made sense to me.
For the most part I generally see pro photogs...and I don't see a lot of them in my particular daily routine with Canon equipment. I think Canon in particular but also Nikon are so firmly entrenched in pro ranks that it's not likely you'll see Pentax back there. My opinion.
Check the Olympics...or some big time news conference. It's invariably Canon or Nikon.
But I tell you...the way these cell phone cameras are improving...I wonder if in 5 - 10 years at press conferences (not sporting events) if things will drift that way. In the '60's we carried cassette tape machines for quotes, now many reporters use an app on their phone or the tiny little digital recorders. Technology marches on and I for one can't foretell the future.
So maybe in 10 years time, if there are still newsprint newspapers around, maybe the press photographer will reach for his smart phone, rather than reach into his Domke bag for his full frame digital Canon SLR.
Who knows...things change and I ask you...when was the last time you saw a press photographer wrangling a Speed Graphic when the local mayor comes to the podium to talk about city sewage issues.