I think they realized streak breaking is a serious problem for people (that people only discover when they're about to break it; I can confirm my wife had this very situation with Duolingo) - and figured out they can make money off it. AFAIK, freezing streaks is tied to in-app purchases somehow, or otherwise burns some gems or points you can replenish with money.

My own experience in trying maintaining chains of habits confirms it too - once I slipped on a longer streak, I immediately thought "screw this, I'm not doing this anymore" - because my mental set point moved from "doing something" to "I need to do a streak at least as long as the previous one, and man, how long will that take...".