It is not a remake.
Remakes take time from designers, this was an engineering project, and one that benefits UC4, as it's on the same engine, and now the same platform.
Porting, or remaking, any other ND game wouldn't have served the purpose this did. And if they did this in three years, they'd have to do it cheap or bundle it with other games. The fact it's only a year old means it's inline with 'GotY' editions of games we frequently see.
Also, given Amy Hennig's departure, Druckley are the unchallenged and freshly crowned Kings of Naughty Dog, this release serves to inform owners new to PlayStation what it is that Naughty Dog do.