It wasn't well executed. The standard migration tool (2to3) was kind of considered a failure and the better one (six) wasn't ready, or even around at the time.

Python came this || close to dying during the migration, its users all having moved to other languages. Primarily data science saved it and then some time passed and libraries moved on, etc, and after a while the cost benefit calculation started swaying towards Python3, probably after 3.3 at least, so 4 years after Python3's launch.