It's easier to land when your opponent is trying to KO you, so they loosen their defense by focusing on offense.
Pacquiao ran through Algieri though in his most dominant/easiest victory in over 7 years.
Rios was barely opening up to throw punches breh.
If he would have tried to actually push Manny back and throw a meaningful punch every now and then like Algieri did, he would have gotten lite up even more. Dude was hurt a couple times just being a turtle.

In the end it's

