The Attitude Era made sense it it's time, especially when WWE was fighting with WCW. These days, PG makes more sense. TNA can get away with not being PG because their only tv shows are on SpikeTV, which is little watched, and already has a reputation for violence and raunch. TNA doesn't have the big merchandising deals, advertising, and so forth that WWE has. While the WWE going back to being R-rated would undoubtedly please some of the more vocal fans, it would likely cost them some of their younger fans, or those not interested in a more violent, raunchier product. They would start losing a lot of their merchandising deals, and parents wouldn't be as likely to take their kids to PPVs, house shows, etc, where most of WWE's money is made, both from tickets and from kids buying a lot of merchandise.
The time of WWE being R is past. If it goes back to it, it will because they're in a desperate situation and have nothing to lose.