The Dark Lord feared Dumbledore so didn’t want to confront him.
The Dark Lord didn’t dare confront Dumbledore himself because he feared him - that’s why Hogwarts remained safe as long as Dumbledore was alive.
“He was takin’ over. ’Course, some stood up to him – an’ he killed ’em. Horribly. One o’ the only safe places left was Hogwarts. Reckon Dumbledore’s the only one You-Know-Who was afraid of. Didn’t dare try takin’ the school, not jus’ then, anyway.”
- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)
The Dark Lord wasn’t going to attempt to kill Dumbledore himself - they dueled in the Ministry, and though the Dark Lord dueled to kill, Dumbledore escaped unharmed. Even then, he only dueled Dumbledore because Dumbledore showed up at the Ministry - he didn’t seek out the confrontation with Dumbledore, it was forced upon him by the circumstances.
He chose Draco to punish Lucius, and because he didn’t matter.
In addition to not wanting to attempt it himself, he didn’t seem to actually expect Draco to succeed at killing Dumbledore. Assigning Draco to kill Dumbledore was a punishment for Lucius, and an easy risk for the Dark Lord to take. Draco was expendable (unlike himself or more powerful Death Eaters), at Hogwarts already, and if he failed it was no great loss to the Dark Lord. While it’s unclear how truthful Snape is actually being, since he of course has ulterior motives and reframed the truth to Bellatrix, he said the Dark Lord intended him to be the one who really would kill Dumbledore (and therefore the one he truly considered capable of it) but he wanted to make Draco try - both as a punishment for Lucius and because Snape was more valuable and worth protection.
“Looking down into her tear-stained face, he said slowly, ‘He intends me to do it in the end, I think. But he is determined that Draco should try first. You see, in the unlikely event that Draco succeeds, I shall be able to remain at Hogwarts a little longer, fulfilling my useful role as spy.’
‘In other words, it doesn’t matter to him if Draco is killed!’
‘The Dark Lord is very angry,’ repeated Snape quietly. ‘He failed to hear the prophecy. You know as well as I do, Narcissa, that he does not forgive easily.”
- Harry Potter and the Half-Blood Prince, Chapter 2 (Spinner’s End)
Either way, it would benefit the Dark Lord - if Draco failed, he’s punished Lucius for failing, and if Draco succeeded, he’d have Dumbledore killed without needing to risk someone more valuable. It’s also worth noting that if Snape is indeed telling the truth about who the Dark Lord’s choices to kill Dumbledore were, that both of them (Draco, then Snape) are already placed in Hogwarts. The Dark Lord didn’t know about the Vanishing Cabinet (since Draco only fixed it after being given the mission) that would allow Death Eaters into Hogwarts - which would explain why his next choice was Snape, rather than another more expendable Death Eater.