For one thing, it's the fact that recently picked, high-quality buds are very resiny and sticky and often dense. For this reason, they burn very poorly unless absolutely shredded.
When I spent time in Amsterdam it seemed that none of the locals used bongs or smoked joints; they all smoked mixtures of tobacco and pot (called "spleefs" or somesuch here in the US) and looked down on Americans for smoking the way we do.
Of course, tobacco is so ingrained in the Euro culture it's hard to imagine them skipping a chance to smoke it.
Another point is, there is much more hash than weed available in Europe (someone correct me if I'm wrong). The only 2 ways to smoke hash are: solo in a bowl/pipe, or mixed with some leaflike substance and rolled into a joint.
BTW, someone there intro'd me to tobacco+whitewidowwweed+crack. Called it a "primo". Whoa, hoss. *That* was a sparkler.