This allows couples to have time to process and accept the Church’s teachings that they are learning—possibly for the first time in their lives.
Not only do the instructors need to plant the right seeds, but they also need to be able to prepare the soil first! Nothing ever grows on concrete.

We know that a one-day or one weekend course is not good enough for the couples.
It might be "convenient" for them, sure, but what is important is what they need, not what suits them.
Single day or weekend courses cheat the couples. It is like getting these couples prepared for divorce.

Would a priest agree to prepare for the priesthood in a day/weekend class?
Would we feel safe letting a doctor operate on us after one class?

Marriage is as serious a vocation as the priesthood, and as important a calling as a doctor’s. 
Plus it is so attacked and confused today in our society that it certainly deserves more than one or two days of preparation.

The Synod of Bishops has been discussing this exact point in October 2015.
They said that it was nonsense to do marriage prep over just a weekend (the one day wasn’t even mentioned).
Maybe it was enough 40 or 50 years ago when couples had had a good remote preparation within their Catholic practicing families as kids.
It is not the case anymore.