Because he was superb since before then, for four decades now. I read Liar's Poker and was hooked from the first sentence.
Look, Michael Lewis is like the Stephen Gaghan of (semi-fictionalized) socioeconomic zeitgeist: he chronicles concepts paradigmatic of an entire generation, people and personalities that affect or transform an entire decade or generation (usually in or about the US, and often Wall Street). And he nods towards the societal implications with a sense of urgency and inevitability but also dark theater. Sure he Hollywoodizes people and quotes, not dissimilar to how the screen adaptation of Mark Bowden's superb 'Black Hawk Down' merged characters and their story arcs, or how loosely 'War Dogs' (2016) was based on Guy Lawson's 2011 Rolling Stone article [0] and 2015 book [1]. [As to 'War Dogs' and Lawson's writings, there's an obvious similar moral fable to three potheads from Miami Beach becoming in 2007 one of the Pentagon’s largest weapons suppliers, by reselling Chinese bullets to US-occupied Afghanistan, and noone at the Pentagon actually wanted to know where private contractors were getting their supply, as the scale of the contracts became increasingly insane.] If you want to more accurately label Lewis' genre "semi-fictionalized socioeconomic zeitgeist", then do.
To be blunt, people have short attention spans, people in the US only read average 15.6 books/yr or median 4 books/yr [2], and moral ambiguity seems to sell worse than clean-cut tales casting characters as good guys and bad guys, certainly on the big screen. But these writers are still fulfilling an important service. Noone actually sits down and reads Inspector-General reports, CSPAN is occasionally great but usually dull to watch, and what now passes for US mainstream media is hopelessly coopted and long ago abandoned investigative reporting [3]. Writers like Lewis, Bowden, Lawson(, Crichton, Grisham [4]) fill a very real cultural void in interpreting what's going on.
[0]: https://www.rollingstone.com/feature/the-stoner-arms-dealers...
[1]: https://www.rollingstone.com/culture/culture-news/the-comple...
[2]: https://katiecouric.com/culture/book-guide/how-many-books-do...
[3]: https://www.fastcompany.com/90646413/why-breaking-points-wit...
[4]: https://news.ycombinator.com/item?id=33391995