Speaking from the gameplay side, reworks are harder than new champions - likely the champion being reworked has a fundamental flaw that needs to be fixed, but fixing that flaw has a 99% chance of making current fans upset - being broken does help things be more fun a lot of times
There's a lot of design work that goes into trying to fix a champion, but still make it fun to people who liked it originally, or making it better in a way that makes sense in some other direction. Many champions who still need this have a bit of this going on (Irelia, Soraka, Poppy). I think to get a rework done right, it can take double the time of making a new champion from a development and playtesting standpoint - depending on the level of design changes needed (Eve is notoriously long, Katarina and Xin were a lot faster. Heimerdinger and Karma are tougher.
Hope this sheds some light on the issue - meeting current expectations, while fixing problems and providing creative solutions, is actually one of the hardest things I think a content designer can take on.