Had they gone ahead with their original plan as scheduled, then 1/3 of those using Android would have encountered certificate errors while attempting to browse Let's Encrypt secured websites. Since then people have moved to newer phones and the number of effected devices has drooped down to about 6%. There is still a bit more than a year left before the transition which will of course continue to move the numbers in a favorable direction.
Initially, they likely didn’t think they had a choice. The root that had cross-signed them was expiring. (And I wonder if anyone else was willing to cross-sign them.) It turns out that root expirations are handled differently (i.e. ignored) on some platforms, including the relevant old Android.
