In theory a pension is stable, guaranteed income. The employer promises a monthly or annual payment for life, and they manage a pool of money to make sure you get that payment regardless of whether the market goes up or down. People like stability.
With a 401k you take on the market risk yourself. If the market tanks (2000 and 2008 come to mind) then your retirement funds are suddenly worth less and your payments to yourself (distributions) go down. Of course, if the market is hot you can also direct your investments to try and ride the wave. Greater risk means greater (potential) reward.
401k’s also have required minimum distributions that kick in as you get older. If you live long enough you will reach a point where you have been forced to drain the whole thing into your regular bank account. Then it’s time for another plan.