Why Photoshop? Integration with Lightroom or Bridge and support for the Adobe Camera RAW/Lightroom lens/camera profiling system is one reason. And Photoshop/Lightroom have the same head start in the market as Pro Tools of course, and for the same reason. They were the best thing going at the time they were introduced, and if I'm honest I think Lightroom is pretty much unbeatable when it comes to working with a few thousand RAW images. Which even hobbyists can accumulate surprisingly quickly nowadays when it costs no more to take 100 photos than it does to take one, and almost everyone carries a camera dressed up as a phone (Adobe even provide lens profiles for iPhones and they're surprisingly decent cameras).

Third-party plugin coders usually focus on making sure their stuff works with Adobe's stuff because that's where the market is. Adobe's colour profiles are also pretty much a standard as well when it comes to reproduction work. Printers/copy houses know what they're getting and what settings will 'just work' and if they screw up there's no room for them to argue the colour profile isn't really quite what it says it is.

There's also the integration with Adobe's other design and web-building applications for those that do that kind of thing.

The Gimp's very impressive in the same way the Gnu/Linux project is and I've recommended it to quite a few people looking for an inexpensive and good photo editor over the years, but Lightroom is an absolutely killer application for photography once you get over the initial learning curve, which is at least as steep as any DAW's if not more so. And the Adobe Creative Cloud model then makes Photoshop available for not much more and they both tie into each other and Adobe's tablet/iOS apps.....

Adobe's business model has been very clever over the years, and they still add new stuff into their software, refine what it does and keep it evolving and pretty much state of the art. Rather like Cakewalk do really, only Adobe have the PT-like advantage of being the first to get their foot in the door.

I had a copy of PT9 MP that I got for practically nothing.  The first thing that irked me about PT was the requirement for iLok.  The only thing I currently have on iLok are a couple of VST's that I don't or rarely use. 
 
Back to PT.  I tried to use it, even bought a recommended book on learning PT but just couldn't get into it.  I even downloaded a 30 day trial of PT10 which was touted as compatible with hardware other than their own.  It did recognize my BCF-2000 but it would not recognize my "compatible" Saffire Pro 40.  People on the PT forum and Focusrite tried to help but I couldn't get it to recognize the Saffire Pro 40.  I finally got help and how to force it to recognize it from Sweetwater.  After I got it working, I didn't find PT10 any better than PT9 MP.  I uninstalled both the PT9 MP and PT10 trial and that was the end of my PT (mis)adventures.
 

There's not a lot of DAW users amongst my musician friends around here. Most ar not songwriters so just want to play. And for those of us who do use DAW's I think Sonar is installed on most every machine in one form or another. 
There were a lot of Pirates which I think was a 2000 decade thing. So some people had one of each DAW running and lots of back and forth with different local projects was happening. At this point in time a lot of these are now full time Sonar users and have bought the lifetime membership. That was obviously a good move. 
There are I think a few Pro Tools rigs in town owned by hobbyist with money to burn. One guy spent $2,000 to take a Prop Tools course on line!!!! He still ask me questions and has a long way to go.  
 
I'm a Sonar user because it was the first DAW plunked on my lap Via Roland which I was a dealer for at that time. 
I was aware of ProTools but to me that looked like something you needed special $$$$ Hardware for. 
I have tried most all the other DAW's as demos and ( downloads)  and never found anything better that would make me switch. 
Pro Tools is one of the few I've never tried and If I do it will only be so I could kill a mild curiosity  

pwalpwal
fyi gimp is also available as a windows build, goodness knows why anyone chooses photoshop these days

did they add the ability for non destructive editing?
 
thats my only complaint with gimp.  I use it for my own personal projects so I dont need the adaptability and compatibility that folks who design for a living might need.
 
but the non destructive editing would be really nice.  bums me out that I cant go back and tweak a gradient or lighting effect etc as a finishing touch before making things final.
 
im usually only making banners and logos for my own web material though, so I make it work for me.  For someone with the same needs as me....  I dont see any reason to switch to photoshop either