Complex sentence: By the end of the 1990s, however, the now two-party system's credibility was almost nonexistent, mostly because of the corruption and poverty that Venezuelans experienced as oil wealth poured in during the 1970s and the debt crisis developed during the 1980s.
Split into simpler sentences: By the end of the 1990s, however, the now two-party system's credibility was almost nonexistent. This was mostly because of the corruption and poverty that Venezuelans experienced as oil wealth poured in during the 1970s and the debt crisis developed during the 1980s.