Wearing my Scout Leader Hat... Hemp (and its relative Sisal), being a natural product, tends to rot when it gets wet. Whilst this can be managed (eg by tarring it), this makes it stiffer which isn't ideal for what scouts tend to do with it. We want nice flexible rope that will wrap nicely round spars / poles when we do our lashings etc in order to achieve the maximum strength for pioneering projects etc. Sadly there were a couple of incidents a dozen or so years ago where structures collapsed (causing injuries) which were discovered to be because the rope used was rotten. As a result of this many groups and camp sites have been eliminating their stock of natural fibre ropes. If it has been properly cared for then I'd rather use a natural fibre rope, but there are more and more people who don't know how to care for it, so it becomes an issue, especially for campsites where there are many users who will leave the ropes damp and often stuffed in a dustbin where they have no chance of drying, stored in a shipping container that sweats :( As a result it often needs replacing every year, and natural fibre ropes tend to be more expensive than the man made stuff. As has been said, man made fibre ropes suffer from UV degredation, but thats another story...