The Last of Us Part 1 happened because Naughty Dog wanted it to be visually "contiguous" with Part 2

gamesradar.com:

Despite generally favorable reviews, The Last of Us Part 1's harshest critics call it unnecessary (opens in new tab) and, in extreme cases, "fraudulent," (opens in new tab), but the developers say the remake was necessary to create a "contiguous" experience between the first and second game.

In a new documentary (opens in new tab), creative director Shaun Escayg and game director Matthew Gallant go in-depth about the making of The Last of Us Part 1, and at one point they discuss the reason they remade the game in the first place. Apparently, they wanted to create a consistent technical experience for folks wanting to play through the first and second games back-to-back.

"I remember the conversation in the studio being 'wow wouldn't it be amazing if we could play Part 1 and Part 2 back-to-back?'" said Escayg. "[If we could] have people play and follow Ellie and Joel's journey contiguously across both games, but we had this fidelity gap. The first game was made almost 10 years ago. So we sat down and we said, 'well OK, what could we pillage from Part 2, what we loved about combat and AI..."

Indeed, the original Last of Us released in 2013 for the PS3, while its follow-up launched on PS4 in June of 2020, just a few month's shy of the PS5's debut. That said, The Last of Us was remastered for PS4 in 2014, so technically, that and The Last of Us Part 2 are both PS4 games.

"The starting point for it was making the flashbacks in The Last of Us Part 2 and seeing those alongside the events of The Last of Us Part 2, it really sparked this inspiration in the studio of just like, 'what if we could make the whole game look like that, what if we could make The Last of Us look like these flashbacks that we kind of faked to be contiguous