I am not sure about the assumption in this question. I belong to a church that is evangelical and does celebrate the four Sundays of advent. Many evangelical churches, however, have lost a sense of liturgy and the worship it can engender when it is done well with a heart attitude that embraces God's presence.
Worship structures like advent are designed to lead us into an appreciation of what God has done for us in Jesus. When simply repeated without thought it can become an empty exercise, but entered into with reflection and a warmed heart, it can be a corporate way for all of us who believe to share in what God has done for us. This is why my church celebrates advent as an evangelical church. I think they are right to do it. For without his coming (which is what advent means) we would not have the fellowship with God that makes an evangelical an evangelical (that is, one who has experienced the good news). Celebrating the good news of God's special presence among us is the core Christmas story. So celebrate advent and let the goodness of God come in story, song and worship.