Optional memorials, movable feasts, and the Sunday obligation.
What do they have in common?
If you guessed the perception of being optional, you may be right. I’m convinced one of the main reasons Sunday Mass attendance is down amongst Roman Catholics is because many have figured out that practices once deemed obligatory, are often abrogated.
Movable = dispensable.
Today should be Ascension day in this archdiocese, although the feast (and obligation) has been transferred to Sunday. This happens with Epiphany and Corpus Christi as well. On the other hand, Marian feasts that are obligatory are frequently dispensed if they fall too close to a Sunday. I find that odd since St. Patrick’s day will be moved to a more convenient day during Lent, and often for non-liturgical reasons BTW. That said, many Catholics no longer understand they are still expected to abstain from meat on Fridays, although the obligation has been lifted, and any one may perform another penance or charitable act instead - which means many do nothing.
Whatever works for you.
So you see, when obligation so easily becomes optional in Church discipline, well the faithful, who are now days well educated thinkers in their own right, figured out that sometimes they can decide for themselves what is right and wrong. And many decide they do not have to attend Mass every Sunday - especially if they do something nice for someone instead, or refrain from murder or other heinous sins. They are good people - God sees their heart. “Hi God!” (Waves from the heart.)
That’s my theory and I’m sticking to it. Although you are not obligated to agree with me. 
