Why did Nintendo censor so many games?

I noticed alot of NES games and SNES games have quite a bit of censorship copared to their original versions. For example the SNES port of Final Fantasy 4 at the time called final fantasy 2 had alot of things changed, like the spell holy being called white, Alot of the Christian references were removed. As well as references to prayer were removed. Than you look at Maniac Mansion for the NES. In the PC version you can microwave a hamster and there is much more gore. With the SNES version of Wolfenstein 3D most of the blood was replaced with Sweat, and The SNES version of Mortal Kombat needed a game genie to have the same amount of blood as the Sega version. I don't think Nintendo still censors games because the Gamecube and wii Resident Evil games have all the blood and gore as the Playstation versions but why did they censor games so much in the 1980's and 1990's ?