There isn't really any particular reason beyond the fact that I loved the build up in the movie with her in mysteriously in the background before exploding onto the screen for the climax. The fact that we'd never seen her on the big screen before helped make it so exciting. In the unlikely scenario that her guest starring role would be completely under wraps until the film hit theaters, Wonder Woman's surprise appearance kicking Bizarro or whoever's ass would be amazing. And beyond her role as helping in the climax, her world would be left to be developed in her own movies as it should be.
I liked the tease. I just wish the movie surrounding it could have been better. But her solo began production even before BvS hit theaters, so it's not like WB was like "here's Wonder Woman, that's all you are getting."