I would say because of the poor understanding of the history of philosophy. My undergraduate professor always found a way to discuss the history of philosophy in his courses, and could tell you the arguments given by the philosophers from the pre-Socratics to Kant. Compare that with my friend's professor, who ended his lessons on Aquinas' or Descartes' argument for the existence of God with, "So, how many of you are considering going to church this weekend/converting to Christianity/(any other assertion that blurs the distinction between our natural and supernatural ends)"? It's also harder, based on the first principles of practical reason, to caricature views you oppose when you know them accurately.

As to why immanentist philosophies are so popular, it's most likely due to the common view that science did not arise until the Enlightenment, when their seeds were planted.

lawrence89 wrote:

I'm reflecting on the relationship between God and time. I'm not sure if we have to consider the eternity of God as an infinite time or as the absence of time. I wonder if there is a dogmatic answer to these questions, or if the problem is still open. Does thomism have a precise answer? Should we use A-theory or B-theory of time?

Jason Lawrence Reed wrote an article on just that topic. Here's the link.

A good twenty-hour course that gives an overview of Thomism is The Mind of Man: The Philosophy of St. Thomas Aquinas, taught by Prof. Harry C. Veryser, Jr.

Also look at the links in the side boxes on James Chastek's blog Just Thomism. The lectures are presented by Duane Berquist.

Also check out the courses offered by the Institute for Catholic Culture:
 The Angelic Doctor – An Introduction to the Thought of Saint Thomas Aquinas
 Piercing the Darkness: An Introduction to Aquinas’ Summa Theologica

Scott wrote:

Etzelnik wrote:

Either way, for myself, why can't we approach the universe as all a manifestation of God's Will, created and constantly sustained on Will alone, while leaving God's essence untouched?

I don't see why we can't. The problem with panentheism hinges almost entirely on what we mean by "in." As John West says, if the Thomistic arguments work, then God is simple and without parts; any version of panentheism that implies otherwise would then have to be false. But if "being 'in' God" can be construed to mean something like "being an object of God's thought/intellect/will," then it's much less problematic; Scholasticism generally and Thomism particularly already have ways of understanding God's intellect/will that allow it to have multiple objects without compromising divine simplicity.

Scott:

One way of how we can be "in" God is implied in Thesis 3 of the 24 Thomistic Theses states, 

Wherefore, in the exclusive domain of existence itself God alone subsists, He alone is the most simple. Everything else, which participates in existence, has a nature whereby existence is restricted, and is composed of essence and existence as of two really distinct principles.

The exposition of the analogia entis in Thesis 4 seems to follow upon this conception of creaturely participation in God's being.

It also seems that we could argue this same conception of being "in" God from our being temporal beings. Aquinas states that we know what is eternal primarily as interminable while also being a "simultaneous whole" (ST I.10.1). And since the whole is prior to its parts, eternity must be prior to time, and God's eternity must be prior to our temporality. I believe James Orr discusses this in his paper "'Being and Timelessness': Edith Stein's Critique of Heideggerian Temporality".