Missing Mass on Sundays and on Holy Days of Obligation is a mortal sin because Sunday is the first day of the week (not to mention the anniversary of the Resurrection) and Holy Days of Obligation are feasting days, and we are supposed to bring the firstfruits of our labour to the Lord.
How can we bring our firstfruits to the Lord if we are not even present in His house to worship Him? But the Lord says that He will abundantly bless everyone who puts Him first.
So, we are to go to Mass on the first day of the week and offer the first day of the week to God. (Since on the First Day of Creation, God began to give to us the whole world, and on the Sunday of the Resurrection, He also added all of Heaven to that. Out of this abundance that He has given to us - the whole world, and Heaven besides - we return to Him at least an hour of our time, and maybe something more as well.)
And before we sit down to eat a big feast in honour of whatever we happen to be honouring on that day, we should go to Mass and offer our praise and thanksgiving to Him, first, since it is He who provides us with what is necessary for us to have a feast, in the first place - after all, He is the one who provides everything for us, including the air we breathe, and lungs with which to breathe it.