This is an interesting question, albeit one that would very difficult to answer. You can compare the various readings in the various Greek texts but that will not explain why the verses were split where they were split.
I looked up the apparatus for these two verses and there was nothing dealing with this particular issue. An apparatus is a listing of various readings in the various Greek texts to determine where various readings come from.
One possible explanation (a thin one at that) has to do with the English translations. The NRSV English translation puts the phrase in V16. Since many of the same scholars were involved in both this critical text and also this translation based on the critical text they may have tried to keep the two the same.
I looked at 10 English versions in my Logos software and only 2 put this in verse 16. The NRSV put it as part of the previous sentence, thereby explaining how we are to give a defense. Only the Lexham English Bible put it in Verse 16 and also ended the sentence at the end of verse 15. This changes the meaning as explaining how a person is to have a good conscience.
I personally would not have much of an issue with putting the phrase in either verse as long as the punctuation is such that this phrase belongs with the earlier phrase in verse 15 and not the later phrase in verse 16.
The Greek text I prefer, Robinson's Byzantine Majority Textform 2005 (which combines elements of the Byzantine text type (TR) and the Majority Text) has it in verse 15 followed by a colon.