Why can nocturne be played when you meet his requirements on an opposing players turn without ambush or reaction but jhin meticulous killer cant not be after also meeting his requirements?

Short answer: Nocturne's text grants its own timing permission; Jhin's only grants an alternative cost.

The two abilities look similar but do different jobs:

• Nocturne — "As you look at or reveal me from the top of your deck, you may banish me. If you do, you may play me for [A]." His own ability creates the play window. It fires at the exact moment of the look/reveal action, wherever that happens — including mid-resolution of an effect on your opponent's turn — so he doesn't need Reaction or Ambush. He's not a normal "play from hand" at all; the reveal effect essentially embeds permission to play him (to a battlefield you control) right then, with no Reaction window for anyone (FAQ #12292).

• Jhin — "If you've spent 4 or more to play a spell this turn, you may play me for [C]." This is a cost replacement, not a timing grant. It changes what you'd pay if you had a legal window to play a unit — but units can only be played in your own Main Phase Open State unless a permissive keyword (Reaction/Ambush) says otherwise. Jhin has neither (his keyword is Vision), so on your opponent's turn you never get a legal moment to play him, and the "this turn" condition lapses at turn's end anyway (FAQ #10443).

Bottom line: Nocturne's ability says "play me now, here's permission"; Jhin's says "here's a discount if the normal timing rules happen to allow you to play me." One opens a window, the other doesn't.