Because they aren't competing. WCW and WWE were in direct competition. TNA and WWE aren't. they air on different days and on different networks. A wrestling fan can easily watch both companies without sacrificing anything from the either company. The attitude era wouldn't come back if they were competing either. There is no conflict between the companies. They are instead making sure that there are more places for people to work in wrestling. If there were just one company wrestling would be either dead or boring.
Heck, the best wrestling is arguably not even on TV. But on internet PPV from the independents, with younger guys who work harder more than the WWE or TNA guys, trying to prove themselves. My favorite company doesn't even have a TV deal yet puts out awesome matches and feuds.