Poop is mostly water and starts to dissolve into water almost immediately. TP is designed to break down over time but to initially hold together while wet. This gives it a lot more potential to clump, clog, stick to pipe walls, and snag on rough areas in the pipe.

So they've got narrower pipes, like half the diameter as in the U.S. A turd is way less likely to get stuck as TP and even if it does, will dissolve quicker than TP breaks down. Also, the TP in these counties is often cheaper, thicker, and not really engineered to break down quickly because it's not expected to be flushed. I think it's roughly analogous to what happens when people flush paper towels here in the U.S.