Because moms are special and we like to give them this day as a "thank you".
Mother's Day owes its origins to several long standing traditions in Europe and the UK where a specific Sunday was set aside to honor motherhood and mothers. Traditionally the day was marked by the giving of token gifts and the relinquishing of certain traditionally female tasks such as cooking and cleaning to other members of the family as a gesture of appreciation.
While the role of the woman has become less rigid in modern day society Mother's Day (or Mothering Sunday) as it has been called in the UK dating back to the sixteenth century, remains an important day for the honoring of the role that mother's play in the home and in society in general. There is a corresponding holiday for Fathers called, not surprisingly, Father's Day, but most sources site this as a more recent addition coming about in the late nineteenth and twentieth centuries.
While considered by many as a "Hallmark Holiday", ie. one with a purely commercial background, Mother's Day is actually a long standing tradition in the UK and several countries in Europe, and has cooresponding equivalents in many countries, including parts of India and many communities in Eastern Africa and the Far East.