Short haul work for long haul airliners is not cheap. For one, maintenance requirements tied to cycles are quite stringent and cycles amass much faster. Of course, Airbus certified its A330-300 Regional, which might solve some of those problems, however, from what i have found on it so far, it is simply a reduced gross weight version, with a high density seat configuration of 400 passengers. Even that type, has higher ASK cost than an A320NEO and A321NEO. It is extremely hard to beat those (or the Boeing 737 equivalent) on a cost basis, and so far neither Boeing nor Airbus have claimed to have done that. Therefore, any widebody aircraft exclusively for short haul service would make only sense on city pairs where it is impossible to get more slots, which i believe was one of the reasons for the former BA 767 european fleet (similar for the Lufthansa A300). Otherwise it would be cheaper to simply fly two A321 instead, which can transport more passengers as well, with up to 480.
Of course, if the airline has to operate widebody aircraft anyway to serve certain routes, mostly longhaul ones, it can be used for the occasional short haul route as well, although that comes at a cost as again cycles add up faster. My former employer, who did that for quite a few years discovered that fact after quite a few years, when wondering at a top level, why maintenance costs were consistently above projections. Or simply said, the route planners didn't talk to the maintenance department...