Because what Derekm is over-looking, with his mythical warm and cosy tree nest - is how the cavity has been formed in the first place.
The bark will have been damaged at some point in the tree's history, typically by a branch being broken-off (forming a 'rain-catcher), thereby allowing rain to penetrate the wood. The rotting process into the trunk would then have commenced, with the majority of the subsequent rot occurring at the upper surface (by virtue of aerobic bacteria) of the cavity thus formed, with much less rot occurring at it's base due to the water-saturation there - which of course is why the majority of tree cavities are found to have entrances towards their bottoms.
After a considerable number of other occupants, the progressive rotting of the cavity would eventually result in a size suitable for occupation by bees. And so the bees duly move in. But does the rotting process then magically stop ? Of course not.
So the reality is that those bees will be living in a cavity which will be cold, damp and even wet during winter, and only reasonably dry during the summer. But whenever it rains, the cavity will become wet again - for that is precisely why the rot has been taking place, and will be continuing to do so !
By making hives from Kingspan, a completely artifical environment is being created which mirrors those environmental conditions which are valued by human beings. It may be a very clever thing to be doing, but perhaps not particularly wise ?
LJ
