The first is that almost all spells require effectively the same amount of effort - Will, focus, line of sight, a wand, a word (or two). In a duel there's only one objective, and that is to have stop the opponent. In this case alone, there's not much reason to switch between spells, because as soon as one hits, you win.
Second, in stressful situations, there's a quote by Archilochus that is repeated from children's martial arts dojos to the highest levels of military — 'We don't rise to the level of our expectations, we fall to the level of our training.' The characters seldom have a chance to sit and think about which spell they're going to use - only those that have been wizards the longest seem to have large variations in the spells they use, and the series is focused on teens who don't have a huge "spellbook" per se to begin with. Even in the Dueling club they set up themselves, Dumbledore's Army doesn't train their participants to be creative, they teach them how to be effective. Snape uses Sectumsempra in lieu of others as it's his personally created curse, and uses it as a signature, to the point where Lupin could take a single look at George's (lack of) ear, and say "This is Snape's work".
Lastly, the spell that characters choose has a certain metanarrative built in. Voldy uses Avada Kedavra on anything, solidifying his character as one who kills indiscriminately. Potter, the OotP & co almost exculsively use either Stupefy or Expelliarmus (Stun or Disarm, respectively), because they respect the life of the target, even if that target is trying to actively kill them. They could use other spells that have the same basic effect - say, petrificus totalus - but it doesn't give any larger benefit than Stupefy, but takes twice as long to say. Snape uses his Slashing curse that is viewed in a gray area - the "good guys" see it as nonlethal, while the "bad guys" view it as cruel enough. In an actual battle, there's no real benefit to switching spells, save adding larger scale environmental effects (Reducto starting an avalanche in the hall, Voldy Vs Dumbledore dueling in the MoM, Patronuses to counter dementor attacks, et cetera).