The crosscourt topspin forehand is a solid weapon for Nadal. It tends to put many players on the defensive. The most they can do is return it right to Nadal. Djokovic on the other hand has one of the greatest backhands in history. And with it, he can return the cc topspin forehand effortlessly anywhere he chooses, be it cc or dtl.
Andy Murray also has a good counter for Nadal's cc topspin forehand except he doesn't have a dtl backhand like Djokovic's and instead prefers to hit cc.
Here's an example of Nadal's cc forehand followed by Djokovic's answer. Djokovic goes dtl.
Here's another example of Nadal going cc forehand followed by Djokovic's response. Nadal leaves the court open.
Same thing again. Open court...
Nadal was still using this same tactic throughout Indian Wells. Del Potro didn't have the legs or the formidable backhand to do much with it.