Quality ? The 500 was a absolutely horrible sounding mixer compared to the competition, the only reason it gained massive traction was the (quickly abused) onboard beat effects, which no other professional club mixer had and their massive discounts for club installations.In a word 'Quality'
The DJM 500 set the scene a long time ago, then the DJM series .... the rest is history. I have seen DJM 500s covered in beer back in the days when smoking at da club was cool................
Djs riding the faders and pots.... everything still working like a gem.
Most "industry standard" mixers at that time could handle at least a pint of two thrown over them every other week (Rane, Formula Sound, Citronic)
In saying that the CDJ's 500-S (CDJ-700 in the USA) were an absolute milestone and most clubs replaced their old (pretty unreliable) Denon Rackmount players with them very fast.
No other manufacturer caught up for quite a few years and by that time they had made their name as a brand to be relied on in a club environment and a true alternative for vinyl DJ's.
Yes there are other options when it comes to CDJ's, but Pioneer were the "Pioneers" when it came to CD Turntables and after 4 or 5 years DJ's were comfortable with the reliability, layout and functions of the CDJ - these have expanded on, but if you played on a CDJ500 back in 1999, you can pretty much walk up to a CDJ2000 and knock out a decent set with no problems.
Clubs want minimal fuss, as do DJ's. A set of CDJ's and USB key will do exactly the same thing for 90% of working djs' as a controller / laptop setup and its completely interchangeable with every DJ from anywhere and no grief with changeovers.
I genuinely could not see any NI controllers or mixers surviving the abuse of DJ box permanently for very long. When DJ's are playing on their own equipment they tend to treat it with a little more respect than the house equipment.