Anyway, agree with what most people have said about the reasons we generally struggle. IMO the grassroots pitch argument is the primary reason. As Heath etc. have said, pitches at club level are generally atrocious. The players that do succeed often are just the guys with a good eye who can hit a few runs when things go their way; there's a proliferation of block-bash batsman at club level that would never be able to step up to a higher level. It's not a good place to develop a technique.
In Wellington this year I think the first two games of the year were on artificial pitches, but often it's more than that, and besides, early season pitches are woeful, where 100 is often a very defendable score. First round on grass this year I think saw every team out for under 150, with a couple of teams all out for 20 and 30. It might be a bit different elsewhere in the country in places with better weather (e.g. Hawke's Bay), but equally in other areas it's worse. Where I grew up, we didn't get on to grass pitches until December some years, and below Premier club level many teams only played on artificial. In fact, the first time I can remember playing on a proper grass pitch was when I trialled for Under 15 reps.
I also agree with what Shane Bond says about talented players cruising through age grades. Definitely happens; I can think of plenty of people I've played with who had so much talent yet have amounted to nothing. They dominate at age grade level, because they're talented and most bowlers/batsmen can't keep up with them, then they get to a slightly higher level and get found out. There's a real big-fish-in-a-small-pond phenomenom at age group level in NZ imo.