Moderators, feel free to remove this but I think this question is better worded as "Why is trench warfare more infrequent today versus in earlier wars and conflicts?" I am going to direct to a few previous /r/AskHistorians answers which will help some.
An answer here from awhile back discussed trench warfare during the Korean War.
The Iran-Iraq War utilized trench warfare and the idea of such was addressed in this sub back in 2015 by /u/DuxBelisarius here.
The seige of Sarajevo, which falls into the allowed timespan for answers here, also had some trench aspects. See here. Hopefully someone can add more on how this was similar and different to earlier trench warfare.
You can also learn more about trench warfare from this thread here.