I know of a few people who call any DAW "protools", it's kinda like people calling a vacum cleaner a "hoover"
anyway, a few reasons to answer your question...
Protools was the first to market as a professional solution, and it has grown in popularity (notoriety?) ever since
Protools offers insane expandability through DSP farms, large format integrated consoles like the Icon, or Venue.
It has become so prevalent that it is pretty much an industry standard
and when protools goes wrong, you phone one guy, not a load of companies who's products are assembled to build a DAW rig.
Protools can guarantee a particlar number of audio tracks for a given set-up.
Protools also have very low latency (VERY VERY low) monitoring, advanced synchronisation options, etc etc etc.
It has nothing to do with PCs Vs macs. Most big PT rigs I know of are PC-based. indeed, they're going to have to be now, because the new macs don't support PCI cards, which makes them completely and totally incompatable with current protools hardware!
However, I personally find the editing in it to be far inferior to sonar, which is why I mainly use a Sonar set up to edit our multitracks - however, we usually work with 12 to 48 tracks, not the hundreds and hundreds that can be accomodated on a PT system.