Could someone explain why its only a requirement on sundays? (and holydays :))
At the most basic level, it is to obey God’s commandment to keep holy the sabath day. For Christians, that day is honored on Sunday, since that is the day of the resurrection.
The Catechism of the Catholic Church says:2041 The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the very necessary minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbor:
2042 The first precept (“You shall attend Mass on Sundays and holy days of obligation and rest from servile labor”) requires the faithful to sanctify the commemorating the Resurrection of the Lord as well as the principle liturgical feasts honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days.
Jesus suffered and died to free us from the slavery of sin.
The primary place we receive the grace to be freed from the slavery of sin is the mass, (provided we ask for that grace).
Thus, to skip mass is to reject the work Jesus did by His suffering and death.
If we skip mass we are essentially saying that for us, Jesus died for nothing, because we reject the means of obtaining the grace that He died to give us.
That is why the Church makes mass on Sunday mandatory. We should go more often.