+1 Psimet. That was the exact response in my head when I read the OP.

You would be best served on a set of reasonably deep profile rims with 36 spokes. A friend is about 260 pounds, VERY powerful, and swears by 36 hole Velocity deep v's lace to some good hubs (can't remember which now). He uses them for everything he does on a road bike. You could source something like this quite reasonably.

There is no reason not to go 3x on any wheel with 32 or 36 holes. Spoke durability is increased significantly with 3x (it's all about the way the spoke leaves the hub at a tangent). I'm a fan of double butted spokes pretty well all around. DT Competitions are a good choice, as are Sapim Race. The double butting gives good elasticity, which you want both for spoke life and for ride quality.

As for why, I would expect that the low spoke wheels would be prone to broken spokes and/or cracks at the spoke holes, as well as being more prone to rim damage from impacts (depending on rim type involved). They will also likely have what you'd find to be an unacceptably noodly or squishy ride for you.

Big dudes need stiff rims with a lot of spokes. I also recommend 25 or 28mm wheels - the biggest that will work with your frame.

Good luck.