Why ? Well probably because helicopters are always impressive to watch on the big screen, especially since most of them end up exploding!
Older movies are also using helicopters. Rambo III is a great example. In this movie, there are 4 helicopters exploding. They are an important part of the movie since they are used as part of the plot.
There are also several helicopters in the Transformers movies. However, the exploding ones aren’t the most popular ones in the serie! Indeed, MH-53 Pavelow helicopters are actually used as Decepticons. (Blackout & Grindor)
Here is a video found on Youtube, regrouping several movie scenes, in which you can see lots of helicopters exploding, including scenes from Predator, Terminator, X-Men, Independence Day, Rambo III, Blue Thunder, etc.