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Also, the ITM (Mitre 10) Cup and Currie Cup are professional with sides which exist year-round. Players might not get paid much, but as I understand it, you get a 'training' wage which enables you to hold down a normal job and train year-round with oversight from the coaches.
The NRC sides only really exist from August to November; outside of those times the players play amateur state comps (Shute Shield, John I Dent, Pindan, etc). Unless you happen to be a member of a SR side or their EPS you don't have access to pro-coaches; and training is done around work commitments.
As most pro athletes will tell you, self-coaching rarely works as well as having a professional to push you (with one or two notable exceptions, eg, David Pocock!) and pro coaches will work on both skills and fitness, whereas if you have limited coaching access, you tend to concentrate on fitness far more.
NZ & SA's 3rd tier comps not only identify talent, but get them ready such that the step-up to Super Rugby is a step rather than a leap. Players who standout at the NRC tend to still need a season or so to find their feet at SR level as there's still a large gulf between the two. That gulf has reduced as the NRC goes on, but given the comparative lack of resources available for NRC players out-of-season, I doubt that they'll ever close the gap as much as the Mitre 10 or Currie Cup have.