To answer this question calls for a bit of theological speculation, but perhaps one reason why there are so many Trinitarian heresies is that the concept of the Trinity itself is difficult to understand and explain. The idea of one God who eternally exists as three Persons is not easily grasped. Our website has about a dozen good articles on various aspects of the Trinity. The fact that we have a dozen or more articles on this topic demonstrates the complexity of the doctrine and how difficult it is to fully explain.
Some Trinitarian heresies are the result of inadequate explanations of the Trinity. Perhaps the most notable such heresy is Modalism (also Sabellianism and Patripassianism). Modalism denies that God exists in three Persons but asserts that God simply appears in three different “modes” or manifestations. Sometimes God appears as the Father, sometimes God appears as the Son, and sometimes God appears as the Holy Spirit. Modalism seems to be in a somewhat different category than the other Trinitarian heresies that deny the full deity of one of the three Persons.
Other heresies that seem to arise out of a misunderstanding of the Trinity are partialism, which teaches that the Father, Son, and Holy Spirit are all parts of the One God, and they are only fully God when taken together. And tri-theism teaches that Father, Son, and Holy Spirit are three separate Gods. Muslims often misunderstand Christian teaching as tri-theistic.
In combination with a genuine misunderstanding of a difficult concept, Satan is always at work attempting to distort our understanding of God. Where “honest mistakes” end and Satan’s purposeful deception begins may be impossible to tell. Satan’s efforts to distort teaching about God is a second reason for so many Trinitarian heresies. The following heresies seem to be more of an outright assault on biblical teaching than a misunderstanding of a difficult concept, as they deny the full deity of either the Son or the Holy Spirit:
Arianism teaches that Jesus was pre-existent but not fully God—Jesus was a created being. (Jehovah’s Witnesses teach a form of Arianism, and the Mormon concept of Jesus might be considered Arian, as well.) Macedonianism teaches that the Holy Spirit is a created being—not fully God. Adoptionism teaches that Jesus was fully human but was somehow “adopted” by God in a special way. Similarly, Ebionism teaches that Jesus was a human being with special gifts that distinguish Him from other humans—but still human. In the same vein, mainstream liberal theology teaches that Jesus was simply a man who was deified by Christians at a later date.
There is nothing more important than our understanding of who God is. Furthermore, the deity of Christ is central to the gospel. Therefore, it only makes sense that the truth of the Trinity would be at the center of false teaching that relies on human wisdom or is inspired by satanic deception.