Past perfect and past simple

Please look at the following "Trump and his entire campaign team are precisely the kinds of fringe characters that Russians have traditionally cultivated, to no measurable effect. Even the insiders on Trump’s team were outsiders: Jeff Sessions was seen by his Senate colleagues as a crank and an extreme outlier on immigration and other issues; General Mike Flynn had been fired by the Obama administration for insubordination that stemmed from his penchant for conspiracy theories. Others, like foreign policy adviser Carter Page, had never been allowed at the grown-up table before. One-time campaign manager Paul Manafort, for all his supposed Republican/Washington credentials, was basically a paid hack for a succession of the world’s crooks. And Steve Bannon, above all, had turned being a fringe character into a profession." Could you please explain the use of past perfect and past simple. Is the main timeline in the past of in the present? Why speaking about past events does the author sometimes use past perfect and sometimes past simple. Thanks a lot.