In summary, quality over quantity.

I don't have a very high opinion of Skytrax, especially their airline ratings, but in this case I agree with them.

Star Alliance has a lot of carriers, but a lot are also-rans of poor quality. OW made a mistake admitting Air Berlin (without making them increase their quality) but overall all their members are airlines you might well choose to fly on. Star... not so much.

Star experience is not well unified. For example the number of per-airline exceptions on the Star RTW ticket is huge compared to the number on the OW RTW tickets. The infamous United Star-net blocking, limited LH reward availability unless you use Miles&More miles, etc, etc.

OW gives its top-level status members many of the perks of "First" travel (lounge [1], checkin, baggage priority, seat assignment, etc) and particularly top OW status is treated like top same-airline status on the operating carrier. Star gives top-level FF the "Business" experience.

So overall in my opinion OW gives a much more consistent experience across airlines in the experience, and a much higher-level experience overall. Since I, an d I suspect many other OW customers, travel very internationally on several carriers for business and leisure, this is important to me.

[1] Please don't rehash the BA Concorde Room, QR F lounge, etc... too much. OW Emerald still gets access to most alliance F lounges, *A Gold does not.