There are many reasons. The three biggest ones for me are:
1) Many of the characters are derivative of famous DCU characters, so they end up feeling like third stringers when in-universe they used to be the top dogs.
2) Wildstorm took A LOT of risks and some stories had huge scopes that do not allow them to easily fit into the DCU. You’d have to change a lot about some characters and how they operate so they can have a niche in the DCU without stepping on the toes of the existing DC heroes.
3) They haven’t made a true effort at integrating WS history into the DCU. Superman & the Authority makes mention of the Authority’s original run as having take place in the DCU, but it feels very unrealistic. And now they are basically rebooting Wildcats. Things don’t fit together well because there was no real effort.
I think you either need to make the Wildstorm character AU refugees that are now stuck in the DCU if you want to keep their classic tales as they were or you just disregard the WS stories and reintroduce them properly to the DCU, making significant changes.