Why is the aviation industry taking so long to move away from the tube with wings model?
In aviation there are simply configurations that work the best when working out all the compromises caused by competing and contradicting objectives, and these become timeless and continue for many decades until some completely new physics paradigm comes along to upset the apple cart.  

So, the tube with wings is the most efficient way to do it, not from some optimized theoretical single minded objective standpoint, but from the real world need to balance this goal against that goal.  You need to minimize frontal area, and to get the most people in the smallest frontal area, you have to arrange them lengthwise.  But not too long, so at some point you have to make the tube wider instead of longer.  A tube, low in drag, efficient structurally and easy to make, can't be beat.

The various radical approaches almost always fall short in key ways, which is the real reason you don't see them.  That doesn't mean stopping experimentation, because some day someone will come up with a radical configuration that sets a new standard while managing to meet all sorts of secondary needs, and then everybody will start to copy that configuration.

I always like to point out the ultimate engineering sweet spot as the pinnacle of this sort of thing, and really, probably the originator of the "tube with wings" concept - the Douglas DC-3, the greatest engineered conveyance of modern times.  This is a set of engineering compromises in such perfect balance, that eighty five years after it was designed, at the end of the second decade of the 21st century, they are still purchased and operated as working machines to earn a living for somebody, pretty much as they were originally built, as opposed to being operated for show or fun or as a museum piece.  That's engineering perfection, the grandfather of all modern airliners, still making money for people, the original tube with wings.