There are many ways of achieving the same goal. The main reason Pro Tools is popular in professional audio production has more to do with them being the first out of the gate than anything else. From the beginning of commercial software, piracy has always been an issue. Digi was the first to realize that in order to emulate the functionality of a traditional studio, the common desktop computer of the day was incredibly underpowered, so they killed 2 birds with one stone: they made expensive add-in cards adding DSP power to anemic computers to make them do all this incredible emulation and made sure their software wouldn't work without it. And of course this was expensive, and only within the reach of commercial studios and successful producers.
Now take a bunch of audio engineers used to work a traditional audio studio and give them a paradigm that resembles their normal workflow and introduce them to the concept of non-linear editing: that's how it started. Give them a serious head start in reliability and functionality and you basically have a captive audience: after all the investment and time installing, learning and updating this system, they're tied in. Who has the time to learn another system? Pro-audio production can be hectic and time-consuming. Oh, and call it PROTools, so there can be no mistake that it's meant for professionals
(Just brilliant...)
Today the picture is changing. Technology marches on and operating systems like XP and OSX are much more stable and desktop computers have become much more powerful. The need for outboard DSP is diminishing all the time. The variety of programs also reveal the particular biases of their creators. I don't know Samp very well, but Logic and Cubase have more compositional slants, Nuendo and Pro-Tools are more for audio and post-production, Ableton Live for live interaction with audio materials, etc. Everyone know that ProTools has always been anemic in the MIDI department and quite a number people prefer Nuendo's editing style to ProTools. Logic seems lacking in comparison in the audio department. So basically there isn't ONE program that excels at everything. Or there only would be one program in use
But one thing I can't stand is the obviously biased and unfounded bashing that goes on whenever people compare all these various platforms. Theirs is often simply the best. There's some in this thread, but I'm not going to bother... It's like the Apple vs PC commercials: while there may be some truth, it's often unbelievably exaggerated and biased (and PC is just such a loser!), but many are funny, anyway; if only DAW-bashing were so entertaining
BTW: I've got a secret flame for Reaper just because it's rate of evolution is simply astounding and their response to customer requests is unparalleled. AND it's extremely well coded; these guys are good and the code is tight in a way I haven't seen in a long time. It's definitely not everything I need a DAW to be (yet!), but I'm watching it with keen interest.