The main problem with internal routing is that the hard work of doing it properly is invisible, just like the heinous sins of omission that more often occur. There's very little incentive beyond a good work ethic to do it right.

Also, the ways of doing it wrong include not just making maintenance a hassle, but also potentially stuffing up shift quality, an unthinkable proposition with external routing. Giant, of all companies, performed a monumental cock-up on the old Propel, where the shift housing from the bars meets a stop in the top tube, then goes through a couple of lengths of brake housing type stuff, just flopping around in the downtube. This mess, including the top tube stop, is just held in place with cable tension!

What a joke.