Earlier quoted context omitted.
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…
Another reason springs to mind: if airports are congested - as they probably are - if an airline continually wastes allocated take-off/landing slots by not actually using them, the whole airport becomes less efficient and those precious slots could have been better used by someone else. This disincentivizes doing that. I'll leave why someone would hog slots they have no intention of using as an open question.
What they do do though is hold onto slots at congested airports that they perhaps don't have a use for until a few weeks/months down the line.