SWBKCB
Because they could/can only be used on certain routes. The reason they became available was to ensure competition on routes which BA and BMI served once BMI became part of BA. Most of those routes are regional routes.
of course, an airline with a large holding of slots - Lufthansa or SAS for example could buy these slots, shift their slots around and then make a pair which would be better suited to long haul ops and sell them off or lease out that pair while operating these slots on their short haul flight. One way of making it work.