Why Does the Pentatonic Follow the Root Chord But Major Scales Do Not?

As explained in Fretboard Theory, in order to apply major scales properly you must examine a whole chord progression and determine which key all the chords fit into together. Then you can play that major scale over the whole chord progression. But when applying the pentatonic scale, you only need to identify the root chord (tonal center) in the progression and then play a pentatonic key that corresponds to it. Why do you play the pentatonic based off only the root of a song and not the whole key or parent major scale? Why is the pentatonic so special that this works?

The pentatonic scale can follow the root chord because it includes scale degrees common to all possible major scale modes. For example, let's say you have a progression revolving around a C major chord. Well, depending on the other chords involved the C could be the first (Ionian), fourth (Lydian) or fifth (Mixolydian). Each of these modes are slightly different and produce different chords, but they all include the notes in the C major pentatonic.

C D E F G A B - C Ionian (C is chord I in the key of C)

C D E F# G A B - C Lydian (C is chord IV in the key of G)

C D E F G A Bb - C Mixolydian (C is chord V in the key of F)

C D E G A - C major pentatonic

The same thing happens in minor keys. If you're playing a chord progression that revolves around an A minor then the three possible major scale modes are Dorian, Phrygian and Aeolian.

A B C D E F# G - A Dorian (Am is chord ii in the key of G)

A Bb C D E F G - A Phrygian (Am is chord iii in the key of F)

A B C D E F G - A Aeolian (Am is chord vi in the key of C)

A C D E G - A minor pentatonic

You can see that the pentatonic scale is missing the intervals that complete the whole major scale. These same missing scale degrees are what make the modes different. Without them there is no conflict regardless of mode. In fact, if a chord progression changes keys (parent major scale/mode) but still revolves around the same chord, then you can still play the same pentatonic scale over it without needing to consider the key changes.

For example, "Moondance" by Van Morrison has a section based on A Dorian and another section based on A Aeolian. If you want to play full major scale patterns then you'll need to switch keys. But since both sections revolve around the same Am chord you can continue to play A minor pentatonic scale patterns.

Pentatonic scale patterns are easier to apply and require less thought or music theory knowledge for the reasons explained above. This shouldn't be considered a bad thing because often times the sound of the pentatonic scale is a better choice. But a smart guitar player will also understand how to use and apply major scale patterns and modes. Sometimes it's the more complicated approach that sounds best. Use your own judgment.