There are psychological reasons why some are TULIP people (generally speaking), as well as why many aren't. I think that Ruben's observations are quite appropo.
I also agree with the commenters who point to the sovereignty of God in opening people up to receiving his truth. So there are definitely spiritual factors at work.
I want to make an additional proposal that I think is somewhere between both those accurate observations.
A lot of people simply don't accept what the Bible teaches. I know this argument tends to "go both ways," that is, that people who deny TULIP say that TULIP people are ignoring various texts of Scripture that are their go-to verses. However, I don't personally know any TULIP people who actually refuse to deal with so-called "problem texts," or who offer what are transparent desperation moves to avoid the force of certain texts. The Reformed answers to the challenges commonly offered them are not gymnastic, but follow simple rules of logic and argument; in other words, they appeal to the questioner assuming his fundamental agreement with common rules of rational discourse.
I will stick to my guns, and say that when push comes to shove, many folk take the John Wesley option on the Rom.9 passage (for instance), and just say, "I know what it sounds like it's saying. And I don't know what else it could be saying. But I just know it doesn't mean what the Calvinist says it means."
I credit JamesWhite for putting it in just this way as I heard it: that what is hard for men to accept about the Bible's teaching is not typically the truly difficult passages, but that the clear teachings are simply unacceptable. Unbelievers (of the sort made famous by the likes of Christopher Hitchens, a man who must be respected for his unflinching intellectual honesty) are often more willing to accept the Bible's terms than are Christians, because the unbeliever both reads the Bible and openly hates its statements and implications.
Unfortunately, many professing Christians are to a degree (hopefully not to their exclusion from heaven) only willing to accept God's Word on their own terms, and not (absolutely) on his own terms--not on what he has said about himself and about man's relationship to him, plainly revealed on the face of Scripture's witness. That truth hurts.
People are only too willing to latch on to various verses here and there; not bothering to treat Scripture as a single, coherent witness that should be integrated and systematized; and assert a comfortable position that makes men the final arbiters of their destiny. Of course this is a more popular view, in a place and time where objective demands (with their oppressive laws of conformity) have been submerged in a sea of subjective opinions about how reality conveniently conforms to each man's preference.
The actual God of the Bible is Godlike. Coming to terms with him is an actual surrender of one's preferences to his self-witness. Which is, I say, nothing but the fully-orbed whole counsel of God--nothing left out, nothing whitewashed, nothing overlooked or minimized. Which results in a TULIP definition of what salvation actually consists.