>>1637283Have you ridden with one? Those trailers generally suck to ride with because they cause slack action, limit how tight of a circle you can steer, and they can jackknife on fast descents or when braking hard. They're fine for casually riding around and pulling kids, and super useful for carrying heavy loads in certain contexts, but they come with enough drawbacks that you wouldn't want one on a long or spirited ride. Plus, who needs to carry that much cargo when touring? You can put some BIG panniers on most bikes and those do have some drawbacks too, but nowhere near as bad.
As other anon said, apparently the one-wheel Bob trailers are easier to ride with, but that's still a relatively expensive and complicated way to haul more cargo than most tourists need, which is why they're a niche item, not popular.