I've been looking at tourers recently and they tend to have 36 spoke wheels (as does my hybrid on the back after I broke my original rim). What I don't understand is why high spoke counts aren't used on more bikes. Race-style road bikes with low spoke counts and smooth roads I understand. Maybe 26" wheels are tougher than 700C (I don't know). But CX bikes run (sometimes much) rougher surfaces on 700C and similar tyre widths to tourers -- without luggage but surely it's shock load that breaks spokes. Or 29er hardtails -- rougher conditions still, and it's not like you can expect good technique from all riders all the time.
Summing up comments into an answer, including a few points of my own:
- More spokes are stronger; more holes in the rim weaken it. So a sweet spot must be found.
Standard numbers are good, for hole count and spoke gauge
Tourers have to keep going a long way from support, so durability counts (see also brakes); a little extra weight on a laden bike makes little difference unlike in a race.
- One broken or loose spoke in 36 doesn't make the wheel go as far out of true as one in 32 (I've experienced this on my hybrid)
- Although materials have improved, touring is a conservative discipline in which there's no imperative to change what works.
- Touring can mean sitting and spinning over rough roads for hours on end with a lot of weight on the frame. In CX most of the weight is above the suspension formed by the rider's legs and the rider can stand as required.
Touring bikes are often heavily loaded with paniers in addition to the rider. The spokes transfer the load from bike frame to the rim and road. Higher spoke count wheels means that the overall load is shared through more spokes resulting in a lower loading per spoke and thereby lowering the chance of spoke failure. This is more important for a touring bike than reducing weight for performance purposes as the rider may be a very long distance from support or repair facilities.