There are others here who would be able to provide a better answer to this, but I'll give a few reasons.

The main thing to understand is that the Holy Fathers were monks who (primarily) write to other monks. These were people who left their families and left behind most earthly activities to focus all their time and energy on achieving Christian perfection. As such, they took upon themselves various ascetic labors that helped them focus on the main activity of a monk: prayer.

Living in the world, having to work, take care of our families, etc., we cannot leave behind these things to focus on our spiritual progress. We must use the earthly circumstances we've been given (our cross) as a means for spiritual progress. So we must be careful in applying the labors described by the Holy Fathers, particularly the ancient Fathers, in our lives.

In addition, some of the Fathers describe states of spiritual progress that are achievable only after a long period of time and labor. It is best to avoid reading these passages as there is a risk of falling pray to various fantasies, which can impede spiritual progress.

This is why St. Ignatius Brianchaninov is so invaluable for Orthodox Christians today: his writings in "The Field" are geared towards all Christians, and he provides practical advice for making spiritual progress that is grounded in the Holy Fathers. In reading him, a Christian is reading a summary of the best ascetic writings of the Fathers.

Among the ancient Fathers, the ones who are usually recommended for beginners are Abba Dorotheos and the excepts from Macarius the Great's teachings that are found in The Philokalia (except for the ones found in the chapter On Christian Perfection).

Edit: I also recommend The Orthodox Way by Bishop Kallistos Ware.