Why can't you stop Sigma's ult?

[Cerburus437-1424]

Here’s a place to read up on ults how they work and whats what

A guide to Ultimates and their types

In this post I will be covering every single ultimate in Overwatch and what sections they belong to, it has 3 main categories: Transformation type ultimates: This class of ultimates provides your hero with either an enhanced kit, a passive benefit in addition to what you normally do, or a new kit with which you can work with. They cannot be canceled by the effects of CC only by killing the transformed hero. Cast Time type ultimates: This class of ultimates provide a benefit to yourself or …