A diatonic scale (major or minor) has seven notes. The pentatonic scale is a subset of that with only five notes — two notes of the diatonic scale are dropped. It’s exactly those two missing notes that make the pentatonic scale so beginner-friendly and so ubiquitous in blues/rock.
Why the pentatonic scale is so beginner-friendly
The two notes missing from the pentatonic scale are, of all things, the ones most likely to “rub” against a chord — small half-step clashes with important chord tones that would need to be resolved carefully to sound good. Without those notes, practically every note of the pentatonic scale sounds good over the matching chord, no matter where it lands in the bar — which is why you can improvise freely with it right away, without knowing music theory.
When going diatonic pays off
The full seven notes bring more color and tension, but require more of a feel for which note fits when. The 3NPS system is the natural next step once the pentatonic boxes sit solidly and you want more melodic options.