Foreign investors buying condos here

A brief CBC piece examines the condo construction boom and its popularity among foreign investors.

It’s mentioned in passing in a quote from a McGill prof: “Low interest rates, higher disposable income and municipal incentives have made home ownership more affordable and fuelled purchases… However, it has caused an upward pressure on rents.” Can someone explain to me why, if more people are buying these new condos because they can, rental stock gets more expensive? Why wouldn’t the voluntary removal of a class of potential tenants make rents either stabilize or even decline a little? If you’re adding many more living units to the market, converting more prosperous tenants into owners, why would rents go up?