What's the most charitable explanation for why Bill Ackman has significantly underperformed the S&P500 in his fund PSHZF?

I do not own the fund, and am not a huge fan of the guy. Just want a devil's advocate. I guess the idea here is that he's a "brilliant" investor. But clearly he's underperformed a simple index. Why would any one invest in any of his funds? Please enlighten me. Thank you!