Because the Roman Catholic Church declared to have transferred the solemnity from Saturday to Sunday, without any Biblical evidence for this:

Convert’s Catechism of Catholic Doctrine
Q. Why do we observe Sunday instead of Saturday?
A. We observe Sunday instead of Saturday because the Catholic Church, in the Council of Laodicea, (AD 336) transferred the solemnity from Saturday to Sunday.

Convert’s Catechism of Catholic Doctrine, by Peter Geiermann, published in 1928

The Catholic Church also claim they are above the Bible and supposedly proved this by changing the Sabbath from Saturday to Sunday:

The Catholic Record
The Church is above the Bible; and this transference of Sabbath observance from Saturday to Sunday is proof positive of that fact.

The Catholic Record, Volume XLV, 2342, London, Canada, Saturday, September 1, 1923

Another excuse the Catholic Church uses for substituting Saturday for Sunday is that Christ rose from the dead on a Sunday and that the Holy Ghost descended upon the Apostles on a Sunday:

Convert’s Catechism of Catholic Doctrine
Q. Why did the Catholic Church substitute Sunday for Saturday?
A. The Church substituted Sunday for Saturday, because Christ rose from the dead on a Sunday, and the Holy Ghost descended upon the Apostles on a Sunday.

Convert’s Catechism of Catholic Doctrine, by Peter Geiermann, published in 1928

However, the Roman Catholic Church assert there is no Biblical evidence for Sunday observance, and laugh at Protestants who demand the observance of Sunday:

Catholic Church Extension Society (1975)
It is always somewhat laughable, to see the Protestant churches, in pulpit and legislation, demand the observance of Sunday, of which there is nothing in their Bible.

Peter R. Kraemer, Catholic Church Extension Society (1975),Chicago, Illinois.