I imagine a number of factors help with that (in addition to overall game quality):
The spoilers may have put off people who weren't committed to completing it
People felt they needed to complete it in order to have an opinion of the whole game - it was quite polarising, which drove even those who didn't enjoy it to complete it
People were driven to complete it ASAP to avoid spoilers, and to be able to join in discussions without risking it being spoiled.
It was on-rails, and had different difficulty levels and accessibility options, so no real barriers to completion
I know I tried avoiding spoilers and was keen to complete it as soon as possible, come what may. I trusted Naughty Dog to deliver a good game, I was a big fan of the original, and I couldn't have formed a full opinion of its sequel without completing it. So I was determined to see it through, even if I wasn't enjoying it.
"I don’t know how much I want to reveal... [co-writer Halley Gross] and I did write an outline for a story, that we're not making – but I hope one day can see the light of day – that explores a little bit what happens after this game. We’ll see.”
Druckmann says there's been "quite a bit" of discussion internally about a Part 3, but pointed out how much work is involved in making a game like this a reality: "These games take so much to make. [Part 2], I've been thinking about it for seven years, between when the last game came out and when this came out [...] You want to make sure you’re jazzed by the idea that you have, [that] it feels like it’s challenging.
"We’ve now had two games that I feel speak to something universal as well as telling a very personal story for these characters. With one game, there’s no pattern of what a franchise is. With two games, now there’s starting to be a pattern – now I feel like there are some structural and thematic themes you'd have to stick to if you're making a third game."
It seems that Naughty Dog is currently exploring what to do next. "After we finish one of our big titles," Druckmann explained, "we take a long time to explore different ideas, whether it’s going to be Last of Us III, whether it's something new, whether there's some old franchise we want to go back to. I like to fully explore all of those, then look at, like: okay, we have all of these ideas in front of us. As a studio, what do we want to commit to? Because it’s a huge commitment – monetary, time, passion, talent, so you think about all the opportunity costs that come with that."