By the time Wasim reached peak, PCB had shifted focus from useless Test cricket to more important ODI cricket, more importantly in UAE. To stack up points, you need to play lot more Tests & 4-5 Test series - if you look at the top rankers, ATGs, each of them actually has enjoyed few grand series with 5-6 Tests and calendar year with 14-15 Tests. Wasim, never got the chance to pick 25-30 wickets in a series because after 1992, next Series with 4 Test PAK played was in 2006. And, during his prime, PAK in general played fewer Tests, therefore his career stats & ranking in Test is a bit misleading. Between 189 to 1995, had PAK played say 80 Tests, he would have definitely crossed 900 points and 500+ career wickets. Imran also reached pick in a period PAK played probably 35 Tests in 3 years. Last PAK played a calendar year with adequate number of Tests, MoYo did cross Viv's annual grand total of Test runs, therefore ability in PAK players were there; however, PCB confessed from that sin of playing so many Tests in a year and purified themselves from the curse by playing ZERO Test in next year.
That, higher number of Tests in a series or a calendar year condition is not unanimous actually - it depends on the players. For example, if PAK plays 15 Tests in a year now, most of the player will loose ranking points by year end from start of the year, because now opponents are better skilled, better managed, better conditioned, better led and more intelligent - they'll work out QeA products and expose them by 3rd, 4th, 5th Test; and by half way of year, most of T20 heroes will break down. Even Misbah's team lost last Test to WIN, ZIM, SAF, NZ in UAE - once tourist had acclimatized with UAE; therefore at present what you see the ranking of PAK players are actually under their optimum condition; it was opposite for County products - Zaheer & Javed also should have been ranked in top 3 in the years PAK played lots of Tests (I can tell that for Javed in 1987-88 period).