Why are trigger shifters so widely used, instead of the twisting grip shifters(revoshift)?
In the context of a high-end MTB designed to go quickly over difficult terrain, trigger shifters have the following advantages:

They can be operated using only the thumb, without changing the grip, so the index finger is always ready to brake.
Getting mud on the gloves does not impair operation.

Modern grip shifters do exist, Sram makes a wide range of those, and in my experience, the cheapest option (the GX model) is very reliable and performs very well. For completeness sake, grip shifters have the following advantages:

Ability to shift through the entire cassette in one motion (assuming a single chainring setup, typical for the context), useful before a sudden steep hill.
Absence of a protruding element to be hit by the knee in case the leg unclips unexpectedly.