The concern is that capitalism allocates resources to the production of socially valuable goods because corporations can raise money to buy the means of production of those goods before producing them by selling shares in their future profits to investors who are willing to accept some risk of bankruptcy in exchange for those future profits, which result from consumers' rational judgment that the goods are sufficiently valuable to them to be worth buying at a price higher than the cost of goods sold.

None of the links in this chain is perfect†, but one of the weaker ones is where investors are willing to accept risk. A guaranteed deflationary currency is a risk-free way to get a return on your investment, so companies have to offer a higher rate of return, narrowing the set of goods that can justify such investment.

But the same criticism can be leveled at Treasury bonds and (to many people's minds) real estate, so I'm skeptical of it, and anyway "risk-free" is not a good description of Bitcoin.

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† Consumers knowingly buy socially harmful goods such as cigarettes, they unintentionally buy worthless goods because producers lie to them, investors are largely speculators driven by herd mentality rather than rational assessors of future returns, corporations cheat investors due to principal–agent problems, and externalized costs and benefits such as pollution aren't measured at all. Still, it's a damn sight better at organizing economic production than the Great Leap Forward, the Southern slave plantations, or the medieval guilds.