Because “marriage” carries a lot of religious and controversial attitudes by a lot of people.

If you make it equal legally and use the nice, sterile, unbumuming term “civil unions” for everyone, and let people call them what they want, and carry out their marriages in the fashion they choose, everyone should be happy.

Christians can call them marriages and do their religious thing, I’m sure gays will call them marriages too, and will do their thing in their own way. Taxes and next of kin will be equal, everyone wins.

EDIT: I mean changing the term as far as government is concerned. You’re not going to be able to keep people from using the word “marriage” in day to day life.