Because it matches the chainline of the road cranks. On handbuilt wheels, either kind of hub works, because you can make the axle conversion move it sideways to suit, but for OTP wheels you can just drop a Boost one in and it's already right. If you look at my tandem you'll see the 100×15 hub has a 15mm NDS spacer and a 5mm DS spacer to achieve the same chainline as a Boost hub. The build conceit on the tandem is "all Shimano", and I don't think they do a 6-bolt Boost hub, they certainly didn't when I built it.
Because it matches the chainline of the road cranks. On handbuilt wheels, either kind of hub works, because you can make the axle conversion move it sideways to suit, but for OTP wheels you can just drop a Boost one in and it's already right. If you look at my tandem you'll see the 100×15 hub has a 15mm NDS spacer and a 5mm DS spacer to achieve the same chainline as a Boost hub. The build conceit on the tandem is "all Shimano", and I don't think they do a 6-bolt Boost hub, they certainly didn't when I built it.