WWE Hall of Famer Kurt Angle who is a former five times WWE World Champion recently gave his opinion on why WCW and did not work under WWE. ECW and WCW were two of the biggest Rivals of WWE back in the 90s, ultimately WWE ended up in buying both of the promotions.
After WWE bought WCW and ECW they started to run both brands parallelly along with WWE time by time. Their first plan was to run WCW as a parallel show to WWE since 2001. But after the invasion angle failed, the plans were scrapped and we got RAW and Smackdown as two different brands with different wrestlers.

Kurt Angle On Why WCW And ECW Did Not Work Under WWE
On the other hand, WWE tried to run ECW as a parallel brand since 2006 but the plan once again flopped. They even ran ECW as a different brand. But they had to stop it in 2010. There are a lot of reason behind the failures of WCW and ECW failing under the WWE product.

During the invention angle, WWE brought both WCW and ECW at the same time. In the storyline WCW and ECW formed a team against WWE which they called the Alliance. As mentioned above the original plan was to run WCW as a parallel show to WWE.
WWE Hall of famer Kurt Angle tried to find out the errors WWE made when when he recently talked about this matter on a recent episode of the Kurt Angle show. Here is what he had to say on this matter;

“I don’t know, the fans are so smart, they know that the WWE purchased WCW. When there’s a ratings war and there are two companies with two different owners, fans get excited, they want to see that, they want to compare the companies.
“But when one company buys every other company, the excitement is gone. It’s no longer the ‘Real WCW’ or the ‘Real ECW’, it’s the WWE’s version of WCW or ECW. I think that’s the reason why it didn’t work either time.”
Quote via Wrestling Inc.