They didn't ignore him. Fat tails and jumps are just really hard to estimate in finance. More interesting is that the unstated premise for these link-bait title

They didn't ignore him. Fat tails and jumps are just really hard to estimate in finance. More interesting is that the unstated premise for these link-bait titles (aka great headlines) is "If only they had listened to Mandelbrot, we could have avoided all these problems." It is nearly absurd on the face of it, but worth a comment. Like Louis Bachelier's description of stochastic processes in 1900, mathematicians like Mandelbrot have been inspired by financial time series to develop formal descriptions of the phenomena. Sometimes, this leads to a marginally deeper understanding of the object of study. Regardless, mathematicians and other scientists have little interest in developing prescriptions that would help control or reduce volatility. Most observers seem to think that the amount of financial volatility is way too high in comparison to the underlying economic realities. A few, like Fisher Black, have had the opposite opinion and suggested that the prospects of the underlying economy (including human capital) are actually hugely volatile. Virtually all take the structure of markets as a given and assume they are low-friction and generally structured well. My own opinion is that volatility is mainly a function of information starvation in the market. Just look at the poor quality of financial accounting, auditing, and the cherished secrecy of large risky positions and you can see possible areas to unlock information flow that would allow markets to do a better job of tracking "true" value and ignoring chaff generated by the act of trading.