The thing is, while price movement is determined by flows in the short run, in the long run total market cap is determined by relative investor allocations to different asset classes. The "relentless bid" is there because the money supply is growing and more debt is being issued every day, so the aggregate investor portfolio ends up overweight these asset classes and needs to allocate more to equities.
In order for a sustained bear market to take hold, investors as a whole have to reduce their target equity weightings in favor of cash and/or bonds. That can happen if returns to bonds or cash become attractive again (like 2000 with 5y yields >6%), and/or if some specific catalyst makes investors afraid of losing money (has to be much more than "the market is a bit rich", like the bursting of an obvious equity bubble, or a banking crisis etc).
In recent years you've had the combo of no clear bear catalyst, abysmal yields on bonds/cash, and the emergence of constant bullish flows from passive funds and ETFs, robo-advisors, pension funds allocating more to stock, and corporate buybacks. No surprise it's been a pretty much nonstop bull grind, except for the occasional quant / leveraged trend-follower blowup.
