Why do it now? Because it is serious and solemn and weighty, just right for Ash Wednesday. Why not do it every time? Because the eucharist embraces the whole of life, and therefore has many moods. We must rejoice in Christ's victory from time to time as well as mourn for our sins. Plus, in a church where we don't do communion every Sunday, printing the whole shebang of text for people to follow makes liturgy a test of one's reading skills.
Some day, I think we will go to communion every Sunday. I embrace that, but I think we need about four liturgies in the pews for people to get used to. We can pick or choose the rite of the day based upon the season and the thrust of the day's proclamation. I am not one of those who want to "do it different" every time. That way just leads to confusion. But neither am I one to do it the same way every time.
Anyway, everybody loves the old service. I get more compliments on Ash Wednesday than on almost any other service in the year (including Christmas Eve).