Good question. It is usually thought that modern narrow boats follow their cargo carrying predecessors. I think its more likely that many follow the cheap and cheerful designs of the early years of leisure boat designs, boxy, built to a price, right angles everywhere, easy to weld 'sacrificial edge' along the bottom and more. I sometimes think that Springers might have advantages with a bit of a chine, slightly 'v' bottom and with a bit of work at the ends might be quite good. Canals usually have a 'saucer shaped' profile and hulls really ought to have a similar cross section and fine tapered ends with a bit of sheer to look good. Thing is of course that it would cost more to build, more to fit out and there's a good chance that without those flat sides it would be a sod to reverse