Most likely, they want to make more money, and they no longer care how many free users they lose as long as some of them convert to paid customers.

Free users aren't generating revenue for them. In the past, it seemed like businesses had this idea that infinite free users would somehow lead to infinite revenue, but they seem to be moving away from this philosophy.

Storage has been getting cheaper every year. If a business is skimping on their free plan, it's not because of rising storage costs.