Because on the flip side, airlines could keep canceling flights on less popular routes when they're not at full capacity (i.e. not/less profitable). That would inconvenience passengers. Under this regulation they could lose that route to a competitor that is more willing to service customers (or can operate at a lower cost).
EDIT: I found the EU page outlining the regulation here: https://ec.europa.eu/transport/modes/air/airports/slots_en
EDIT 2: This appears to be a global practice: https://www.iata.org/en/policy/slots/