According to Rene Echevarria, who wrote Part II and was interviewed about this in the season 6 wrap-up issue of Cinefantastique: "We broke it as a one-part episode and it took two acts of the script to get Worf down to the prison camp. The end of Act II eventually became the end of Part I. There was basically too much story to tell - and to do it justice, Michael [Piller] said make it a two-parter. With more time on the planet surface we'd be able to build more sets of the compound".
The same article mentions how season six was the season in which they wanted to break some rules anyway. I find that it gives great scope and depth to both stories, Data and Worf. In Part I, both their respective stories inform one another, highlighted in the speech Worf gives to Data to follow his visions and his "father". This advice Worf gives to Data winds up being what Worf himself follows. I like that this two-parter breaks with tradition and is strong on all fronts.