Why did Harry use only Expelliarmus on Voldemort but Sectumsempra on Draco? Also, if Expelliarmus was Harry's go to spell why use Sectumsempra when he could use it against Draco's Crucio?
Harry fights Draco really differently than he fights everyone else.
So okay. Normally when he fights, Harry's go-to spells are:
Impedimenta (slow down)
Petrificus Totalus (paralyze)
Stupefy (knock out)
Expelliarmus (disarm)
Which means his strategy is get away from/incapacitate his opponent as quickly as possible and that's IT. Harry is not a flashy or creative fighter the way Dumbledore, Voldemort, and heck even side characters like Percy are. Which makes perfect sense. Fighting has never been a game for Harry. It's never been abstract. He'll do all these badass things but when he talks about fighting (like we see during the DA segments) he's not thinking about it like it's something interesting or cool. He's picturing going up against a more powerful opponent, probably while injured and scared. And what's the best thing to have in that situation? Four useful, all-purpose spells that you've practiced enough times that you don't have to think about them anymore.
Zachariah Smith gives Harry a hard time for relying on spell as "basic" as expelliarmus, which tells us that in-universe, Harry's stripped-down dueling style might be considered kind of plain or boring. But Harry's a very effective duelist, probably because he is so practical. He doesn't care about showing off, looking powerful, or intimidating his opponents.
... except when he's fighting specifically Draco.
When he's fighting DRACO MALFOY, Harry brings out the fun spells:
Rictusempra (tickling)
Furnunculus (boils)
Leg-Locker jinx
Levicorpus (levitate by ankle)
Toenail-growing hex
And also Sectumsempra, which he THINKS is going to be another embarrassing/fun spell, because levicorpus and the tonail-growing hex also came out of the Half-Blood Prince's textbook, and Harry thinks this third spell is going to be another one like that.
So my conclusion is that Harry just kind of... uniquely enjoys fighting with Draco. He's not trying to end the encounter as fast as possible, there probably is some element where he wants to show off, hit Draco with something new or something clever. Harry and Draco have a dynamic where it's *fun* to one-up/get the upper hand over each other, and duels are perfect for that.
To me this is a very natural extension of Harry's "lets plot hypothetical ways to get Malfoy expelled" (which he does with Ron, for fun) and "lets try to figure out what Suspicious Thing Malfoy is up to now" (one of Harry's very favorite past times.) So long story short - when Harry fights Draco, he fights like a teenager. When he fights anyone else, he fights like an auror.
