I started using Pro Tools because I worked in a studio that was using Pro Tools so I had to know it. Then when I decided to purchase a Mac and choose a DAW software to use at home, it seemed to make sense to get Pro Tools but back (1998) that would have meant spending at least $10k on a TDM system, so I looked elsewhere. All the pros I knew were using Digital Performer, but one friend of mine showed me Logic Platinum 4, and I liked how the interface looked, so I decided on that. So it's not like an insane amount of research went into that choice.
My guess is, many people end up choosing their DAW that way. A friend uses one DAW and recommends it. Or they read an interview from an artist they respect who says they were using one DAW.
Once you've spent some time learning a DAW, it becomes difficult to switch, so you really need a good reason to switch.
For me, going from Pro Tools to Logic Pro was hard. I remember sitting there, wondering how you could select parts of regions, something trivial in Pro Tools. You just... couldn't do that in Logic Pro. There was no Marquee tool back then. You had to cut regions into parts. That seemed so counter-intuitive. But I stuck to it and learned Logic Pro really hard and used it intensively for one full year before I went back to a studio where I had to use Pro Tools. And guess what, I could no longer work in Pro Tools. I had become so accustomed to the workflow in Logic Pro that I was now lost in Pro Tools.
And yes, there are things that Pro Tools does better than Logic Pro. Pro Tools was created to replace tape machines, and it has effectively replaced tape machines in pro studios. An engineer who used to punch in, punch out, do count-ins, keep re-recording a band from the same spot multiple times etc... on tape could do the same with ease in Pro Tools. Not in Logic. Now Logic has come a long way in that department, but it still does not behave like a tape machine, or like Pro Tools. There are still things that a PT user would struggle with in Logic.
And the other way around is obviously true. Logic was created as a MIDI sequencer. There was no audio at first. Its time realm was always bars, and beats, and divisions, and ticks (where Pro Tools was thinking hours minutes seconds), and Logic thought in terms of BPMs. It's a composers tool first. And it still feels that way.
Walk in a major recording studio, today and chances are you'll have a Pro Tools rig in the machine room, where the tape machine used to be, and chances are you'll see a producer in the control room, with a laptop running Logic Pro hooked up directly to the SSL board.