Probably price. $800 is pretty steep for the entry version, when you can buy a better equipped Lenonvo ThinkCentre for $200 less.
Does that Lenovo have Mac OS?
Obviously it does not. And that's a pity. Whether or not that justifies a more than $200 difference is up to whoever wants to use it.
A Lenovo ThinkCentre is like 10x bigger than a Mac Mini. I wouldn't want one of those sitting in my living room. And my wife would absolutely forbid it.
The only real competition I see for the Mac Mini is the little computers from System76. But they're about the same price.
My Lenovo ThinkCentre M710q is almost identical in size to my old (very) Mac Mini.
I just customized a Lenovo ThinkCentre with a Core I7 (I’m assuming it’s the six core), with a 128GB SSD, 8GB of RAM, Windows Pro, and a WiFi card and the price was $979. The equivalent Mac Mini is $1099.