While there is general research that is used to show a factor in good light, the facts are that there are better and worse times to be exposed to the factor.
In the early years of ETF growth (post 1999), there were not too many variations from a simple theme. The first equity ETFs were just replications of large index baskets, particularly the S&P 500. Those large indices were, of course, calculated by just adding up the market value of each stock. Thus the organization of them was “market weighted.” And soon, those baskets were sliced into their component sectors, like energy or finance and they were also market weighted.