Lots of reasons.
Even as a stand alone sim its pretty good, with good visuals, sounds and pretty realistic physics.
But at the time of its release it had the best online racing code I'm sure GPL'ers might disagree. Easily 60 drivers from all over with little warp / lag.
Sierra didnt release any updated versions so many modifications add on series sprung up.
Hundreds of tracks, at its peak I had 150+
Dozens of full modified series available, with cars, sounds, physics.
I had a few of these, Craftsman truck, TPCC, TPCC2, Race of Champians, Open wheelers, around 11 series in all.
Popularity coincided with the growth of RSC so there was a place to develop and release new mod series, and many other new bits and pieces, paint job assist, sound & other develpment tools and bits.
Also there was Aussie leagues and severs.
And there was nothing else as good after, Rfactor NACSAR came close, but not 100%.
Then the develpment team who "owned" it realised there was a quid to be made and pulled the pin and entered into legal action to prevent any further mods to be made or released.
A long spitefull campaign was fought for a while at RSC / USpits.
iRacing / first racing developed from that. They have developed a pay to play system due to be released soon
IT also has a potted history of Papyrus on page 77
Details in the latest
http://www.autosimsport.net/
