When the West twins were introduced, the franchise was coming off the poorly received Bart run. Mark Waid's returning seemed like a huge deal for about five minutes and a promise to return the Flash to its prime. Then the book debuted, and a lot of focus went to the kids. This rubbed some people (not me) the wrong way. To add isn't too injury, at the same time there was a huge falling out between Didio and Waid, meaning that Waid left the book before the f first issue even saw print. And then after Flash Rebirth the kids were unofficially used to keep Wally of the table.
So lots of people grew a dislike towards them.