why wasn't varus updated in the adc update?

They changed his Ult a bit, but he's a unique, situational poking ADC. They chose the ones they did to make all the ADCs feel more different from one another and have them build different items/masteries. Varus was already different enough from his blight mechanics, they just wanted to make that mechanic a bit more prevalent.

he was his ult applies blight stacks now

He got a change of purpose of his ultimate which now serves as a better initiator at the cost of damage (Rooted ppl will get a full stack of Blight during the CC) but yeah I was expecting some more attention to him. He isn't in a bad position (since he has inbuilt GW and %maxHP damage), still it feels like he ended up with the short end of the stick again.