Why is a hades port impossible? I've see asking posts the same but the comments were just no. I'm just curious why? Is it because it's too modern? But wasn't hollow night new too?

You asked a forum, got "no," then asked a human. Iconic.

You're right that age isn't the reason. The real reason is how each game was built. I'm assuming you mean a fan or homebrew port to an older or weaker device like the PS Vita, PSP or 3DS, since that's where these "no" threads usually show up.

Why Hollow Knight could be ported

• Built in Unity. Hollow Knight runs on a mainstream, off-the-shelf engine, and older versions of Unity could export to devices like the PS Vita. Modders can pull the game's assets and scripts out and rebuild them for that hardware. That's how the Hollow Knight Vita port by PatnosD got made.
• Some "ports" are really remakes. For example, the homebrew PSP port of Hollow Knight is a GameMaker project file, so it's a fan recreation, not the real game.
• It's a 2D game that doesn't need much. The art is hand-drawn sprites, so with enough tweaking it can squeeze onto weak hardware.

Why Hades is a different story

• It uses Supergiant's own engine. Hades runs on custom C++ code with Lua scripting, not Unity. No public tool can rebuild it for a new device, and the source code isn't available. A port would mean rewriting the engine from scratch, which is closer to remaking the game than porting it.
• It needs a lot more power. It looks 2D, but it's rendered in 3D with lots of lighting, particles and effects, and the PC version asks for about 4 GB of RAM. The Vita has 512 MB, about an eighth of that.
• There's a huge amount of content. Hundreds of thousands of words of voiced dialogue and a big pile of assets would have to fit in tiny memory and storage.

So the short version: Hollow Knight was built with a common engine that could already run on that kind of hardware. Hades was built with a custom one that can't, on hardware that's too weak anyway. Supergiant did make official ports to Switch, PlayStation, Xbox and iOS (through Netflix Games).

Those forum comments could've said all that instead of just "no."