For the first time since 1990, hip-hop doesn’t have a top 40 song on the Billboard 100 chart. SHIFTER’s Kevin Bourne offers four reasons why. The hip-hop world is reeling from the news that for the first time since 1990, hip-hop does not have a Top 40 song on the Billboard 100 chart. after Kendrick Lamar and SZA’s “Luther” left…