Depending on time of day/location/etc., breathing fire could attract unwanted attention to the dragons. A couple of scrawny knights could be more hassle to kill than they’re worth, so the dragons might try to avoid breathing fire if it’s dark (and will therefore be a hugely obvious beacon to any dragon-killing people out there).
There’s also a risk to the environment - if they have a particular favourite hunting ground (I’m imagining deer in a forest), they might want to limit the risk of setting the whole forest ablaze, and ruining their easy food source.
Fire breathing could also have a sociocultural weight to it. It could be seen as a bit of a cop-out way to fight something small (you wouldn’t waste your fire on deer/humans, but would save it for another impressive dragon or gryphon or something), and you wouldn’t want to lose credibility by having other dragons think you needed to use your fire to hunt down a tiny sheep.
Or perhaps fire could be part of a mating display? As part of a ceremonial hunt (gathering food for your mate), fire is appropriate. Making cool patterns in the air while you do some fancy aerobatics - definitely sexy to another dragon. But just using it needlessly on other occasions? Kind of takes the excitement out of it a bit.
EDIT - I thought of another thing, so I hope it’s OK to add to my post:
I’m going to make one assumption: that your dragons are intelligent beings, not mindless beasts. If so, they would be able to exert choice over their use of fire, and it could easily be construed as a marker of civilisation in dragon communities.
To make a (perhaps odd) example, humans swear. For many reasons, at many times, which are generally judged to be more or less appropriate. If you aren’t able to ‘moderate your language’ in certain social contexts (you’re serving at a restaurant/in a big corporate meeting/etc.), you risk being seen as ‘uncivilised’. And yet, used in particular contexts, swearing can add impact to humour, force to arguments, emphasis to exclamations, and so on.
If you treat fire breathing in a parallel context, it might explain why dragons would or wouldn’t choose to breathe fire. If you’ve got a rebellious teenage dragon, they might breathe fire all the time. If you’ve got a dragon who’s lived for millennia, they might choose to breathe fire only against a particularly worthy opponent, or in a particularly significant moment of display. Overuse might seem to be a sign that you’re lacking in culture, and intelligent dragons might care a lot about social prestige.
If a dragon is hunting/fighting a creature, they know that they can use fire, but the thrill of the chase, the battle of wits, the physical challenge of the event may be considerably more intoxicating an experience than just resorting to fire.