It is a matter of demand and supply. There is more competition as a pure batsman in the top 5. Being an all-rounder hands him an advantage of being unique. He has a higher chance of getting selected as a mediocre all-rounder (he is quite decent actually, but not at par with those from other countries) than as an excellent top-tier batsman (1. which he is not, 2. Fawad, Haris, Umar Akmal, Fakhar, Maqsood et al are ahead of him).
As an all-rounder his only competition are Hammad, Anwar and probably Ali Khan and Fahim Ashraf. Pace bowling all-rounders are essential in today's limited over games. Australia has Marsh, Faulkner (and Watson). New Zealand have Elliot and Anderson. South Africa have Weise, Morris (and Mc Laren). India have Dhawan, Pandya (and Binny). England have Stokes and Woakes. The intentions of top-ranking nations are evident even if their respective all-rounders are not delivering.