sort of both, i think. because of how they are set up on the fretboard, they are very easy to remember and improv on, thats why they are really popular with beginners. more talented players usually experiment more, branching out into other scales, getting their own sound.

another reason pentatonic scales are popular is because of their versitility. theres a whole lot you can do with it, because of both its location and its spacing. it has a characteristic "rock" sound.

thats my two cents.