Systematic Approach and Low Concentration- Equity Reading

Why do we say that low idiosyncratic risk along with low concentrations indicates a systematic approach?

I can’t get the link to the two

Assuming a large, broad market index (e.g., S&P 500, Russell 3000, Wilshire 5000), a systematic approach will have low idiosyncratic risk and low concentrations. The converse isn’t necessarily true, but it’s quite likely.