Why are 36-spoke wheels a touring thing? 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'd already experienced this on my hybrid; on the tourer I broke 2 rear drive side outer spokes (damaged in an incident with the chain) and still got home with a nearly true wheel.

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.