It simply in accordance with the statement
इतिहासपुराणाभ्यां वेदं समुपबृंहयेत् (१.१.२६७)
As it seems from the face value of the above quotation from Mahabharata, the purpose of Itihasa and Purana-s is to propound the theme of Veda-s (here it is plainly taken as to mean Vijnana and Vairagya) [than to account for the history of the kings]. I don't want to comment any more as it would draw undue arguments.
As per another reading वेदार्थमुपबृंहयेत्। बिभेत्यल्पश्रुताद् वेदो मामयं प्रहरिष्यति।
the reading of Purana-s and Itihaasa (Mahabharata) enlarges/reinstates the scope Vedic learning.
--
Dr. Hari Narayana Bhat B.R. M.A., Ph.D.,
Research Scholar,
Ecole française d'Extrême-OrientCentre de Pondichéry
16 & 19, Rue Dumas
Pondichéry - 605 001