@SuneAuken

I don't know... this is just my personal observation.

The open strings are EADGBe. Some scales don't include some of those notes.

I realised it when I tried to learn the scales of A, C, D, E, and G.

C includes all the open strings
D includes all the open strings
G includes all the open strings
A doesn't include G
E doesn't include D and G

No other (major) keys include all the open strings.

The relative minor keys are the same, only Am, Bm, and Em include all the open strings.

@SuneAuken

For any major key (in standard tuning), there are 3 "bars" where all the notes line up.

In G major the notes at the fifth and seventh frets are all in the scale, as are the open strings.

These are the minor 2nd, 3rd and 6th.

If you're playing in a key where the 2nd, 3rd, or 6th fall on the open E string, all the open strings will be in the key.

This is why keys of G, C, and D work so well.

Relative minors Em, Am, and Bm have root notes on open strings (the B is high though).