Curious as to why this list is not posted in bold for everyone to see at every Church?
Such a list is not very useful, for the following reasons.
While it is true that grave calumny is a grave sin, it is unrevealing, since one still has to determine if the calumny is grave. The same is true for grave scandal. Saying that serious Theft is a grave sin is like saying that grave calumny or grave scandal is a grave sin. A determination still needs to be made as to what constitutes theft as a mortal sin versus theft as a venial sin.
Idolatry is a grave sin, but the forms of idolatry prevalent today are not explicit idolatry (the worship of idols) but rather exalting material things (money, status, power, etc) as if they were gods/idols. So a determination is still needed to determine if the sin is idolatry, or merely some excesses or poor judgment with regard to material things.
It is true that sexual sins such as fornication, masturbation, and unnatural acts are grave sins, and yet there are some Catholics who claim that these types of acts are moral in certain circumstances, with certain intentions.
Lust is a grave sin, even in marriage, but many Catholics do not know what that term means, or how to decide if an interior act is lust.
Pornography is not an act; a sin is an immoral act. There are various types of gravely sinful acts associated with pornography.
Abortion - direct abortion is a mortal sin; indirect abortion is sometimes moral. The use of abortifacient contraception is the mortal sin of direct abortion, and yet some Catholics justify its use, for certain intentions, in some circumstances.
Deliberate failure of the Sunday obligation or Holy Day to attend Mass. The obligation to worship God and to keep holy the Sabbath is a grave obligation, but failing in that obligation only on occasion, without a just reason, is not a grave failure, and so the sin is venial. Also, if there is a just reason, then missing Mass is not a sin at all.