Rylais wasn't as intergral for most mages as duskblade, and it wasn't a flat out nerf as it's costs got reduced alot and the slow barely changed for dot/minions.
So Brand for example recived the same utillity earlyer in the game at the cost of some lategame ap.
Most other APs had enough other items to fall back on, like Luden's which replaced rylais on some like orianna and syndra. The nerfs to rylais didn't have much impact on the performace of mages overall.
For assasins however, raw burst is intergral for their efficiency, their entier damage is balanced around haveing that proc on top. and duskblade gets barly any compensation for the damage it looses, while assasins also have no alternative items to built instead.
They lack the fallback options that mages had both in playstyle and items.