Why do the USA guys have stock class?

[chrispattinson]

Just been reading a thread discussing race meetings, and it was mentioned that in the USA, there is stock class and mod class for most RC disciplines.
After looking online, it seems this is common throughout the USA.
There are specific speedos out just for Stock class, which maximise the slower motors performance through timing advances.
Why do these two classes still exist? I understand when brushed motors were around, it wouldve been a cost / maintenance thing.... but 'mod' racing is so accessable now cost wise, it seems strange to me that some people opt to keep the stock class running??