There are a number of reasons for it. Hopefully I can satisfy your curiosity with one of the following scenarios.
One could be that the drummer uses them as a practice kit at home. Limited space, neighbors, and convenience would account for one having an electronic kit.
Another, the electronic drums often times are ran through a midi and an effects loop. The drummer can give any drum virtually any sound he could possibly desire. Anything from a rain drop falling in a bucket, to a car crash, or explosion. That can add a lot of dimension to a song.
Or, it could be a matter of time and weight. It may take the drummer a lot less time to set up an electronic kit, as opposed to an acoustic. And I am guessing that the electronic kit is a lot lighter to carry and move around. It takes me about 45 minutes to an hour to set up my acoustic set. I do not own an electronic kit, but, I would think set up time would be close to half that time. And let me tell you, they are heavy. My cymbals alone are about 20–30 pounds.
All of those reasons on top of not having to replace heads nearly as often, and drum sticks would last quite a bit longer as well.