my question is "why choose urethane instead of mineral oil or min oil/ beeswax combo?"
my guess as to the answer is aesthetics with less work. it will look nice, but it doesnt sound like the right choice to me. seems bad for the health of the board and bad for the health of the knife.
I think you nailed it. The downside of the oil/wax combination is it has to be re-applied. It would be SO much easier if there was a 'one and done' application for end grain butcher block. An application of varnish would probably be faster and less expensive than soaking in mineral oil and buttering. The other advantage of a hard/curing finish is it tends to be a bit shinier, which makes the board look a little prettier out of the box (not sure about after years of being chopped on). So a varnish has the advantages of being faster to apply, offering a semi-permanent moisture sealant, with a nice initial aesthetic. Some of the potential downsides are... eating varnish, potential issues with the varnish inhibiting bacteria wicking (one of the main benefits of an end grain butcher block), chopping on varnish, eating varnish, and eating varnish. I'm not a hand-wringer when it comes to food safety, and there's a common trope in woodworking circles that, "all top coats are food-safe when fully cured." But I'd argue that just because I can, in theory, eat a thing without getting sick doesn't mean that I should.
As an aside, one of the celebrities in woodworking is a furniture maker named Marc Spagnolo. Super talented; great teacher. He puts out a ton of video content. He did a cutting board video back in 2006 in which he described the pros and cons of a traditional oil/wax finish versus a wiping varnish finish. Some woodworkers took that video as license to varnish wood cutting boards, which probably wasn't the best advice. The vast majority of professional cutting board makers stick with oil and wax.
I wish there was a better mouse trap... but there isn't one. It would be easier to go a different route, but I don't believe it would be better.
Podzap, I hope your butcher block gives you many years of good use and meets your previously defined criteria of the 'best.'