Are you talking about passive investing as in investing in index funds instead of managed funds or simply the general public's increased interest in investing?
Why does passive investing become so hot? ---> Why does trading become so hot? you will find lots of passive and active investors in eliteinvestor.com and not here.
Based on recent history, passive investing can make you 10-20% per year with no significant risk and no effort... what's not to like?
Because since 2008 you could have simply bought and held and outperformed practically every actively managed investment fund out there - including many Hedge Funds. It won’t last, of course, but that’s the reason in a nutshell.
If you invest passively, buy an index ETF or even better dollar cost average into one. You will get very close to the same return as the index. Apparently that's a superior return than most actively managed portfolios. The bonus is you don't have to do any work, make any decisions or take responsibility for your performance.