Few powerful forces oppose legal child porn. Very little money is on the line with CP and very few powerful people would be diminished if CP was legal.

To outlaw or end programs like PRISM you'd have to take on the entire political establishment, the military industrial complex and all branches of the judiciary and law enforcement.

It'd be far easier to legalize CP than do away with PRISM and the like.

This is the right way to analyze the situation, except we must remember that CP is of instrumental value to the legal racket, which is closely allied to MIC. CP gets them in all sorts of dubious positions, and spending lots of sweet taxpayer money, that they otherwise couldn't justify. "Legalizing CP" wouldn't have the effects we seek on the legal system as a whole, because they'd substitute some other issue (if one had to guess, "intellectual property" probably has the interest of various power players...) so they could continue business as usual.