The personality difference is, I believe, crucial.
Not only is Linus a benevolent dictator, he just wants "the kernel to be the best". He has no strong feelings as to what it should do or how it should do it, just that it should do it the best way possible.
Plus, he's a pragmatic engineer. "What's the point of error handling code in unusual situations? If you hit a hardware fault, just crash because there's no guarantee your error handling won't fail too!".
Yeah, I know it's not that simple, but he's happy to make engineering trade-offs, while most of the BSDs aim for mathematical perfection.