Because if we love someone, we should honor him with both our souls and our bodies.
Why do we dress up for weddings, funerals, and certain jobs? To honor those whom we love or those whom we serve. In the Scriptures we read: “Worship the Lord in holy attire” (1 Chronicles 16:29, Psalms 29:2, 96:9). And, the Catholic Catechism teaches that our gestures and our clothing “ought to convey the respect, solemnity, and joy of this moment when Christ becomes our guest” in Holy Communion (CCC 1387).
Clothes say something about what we think, what we value. They also influence how we behave and feel. Sunday is special, and God's House is special. Some things are really important, and our clothing and demeanor ought to reflect this truth.
The exterior reflects the interior, and God definitely deserves the best we have inside and out! This is also why altar dressings, items for the Mass, vestments the priests wear etc., are normally the best we can manage … made from expensive materials and adorned with gold, silver and jewels. However, we have to remember that our fancy dress clothes should be reflecting that our soul is just as dressed up.
Dressing properly for Mass means nothing if we have no love in our hearts.