It seems that most orthodox converts you meet are also liberal Jewish converts. When we liberal converts begin an orthodox conversion, we all seem to think we're crazy to be going from a liberal movement to orthodoxy. But no, you're certainly not alone.
I had the great pleasure to meet with an amazing rabbi who has worked with many converts. He estimated that 60% of the converts he had met were originally converted through another movement. Personally, I would place the number at 2/3 (66% for you math-challenged types), but I think he would have a more educated opinion! Who makes up the other 40% of orthodox converts? My guess is that many are already considered Jewish by other movements, either through patrilineal descent or a maternal liberal conversion prior to birth. I'd like to see some statistics on this one day. But then again, I have an unnatural love of statistics.
Why do so many orthodox conversion candidates have a prior Jewish conversion?
When you think about it, it's a lot easier to "ease into" being Jewish through beginning in a liberal community. It's definitely less culture shock! Going from being a secular person to being an orthodox Jew is a lot like going 0 to 90 in 2.5 seconds. I can't help but wonder about their sanity! Of course, I suppose I'm no one to judge. I started in an orthodox community, and then since I felt I couldn't measure up to orthodox standards, landed in a liberal community.
On an intellectual level, I think there are real philosophical issues that cause so many converts to seek a second (or third!) conversion, whether we realize the reasoning or not. I myself didn't realize the philosophical misgivings I had until someone actually explained the philosophy to me! Then I suddenly knew why I had felt so uneasy. Converts tend to be very curious and somewhat nerdy. Also, as a necessary side effect of conversion, we learn a lot more about our chosen movement (and Judaism) than many of our community cohorts. If you ask the average Jew-on-the-street (of any movement!) what the differences are between the reform and conservative movement, I think they could only say, "More English/Less Hebrew?"
Basically, I believe that it's the philosophical differences that lead liberal converts to orthodox Judaism. While we can't always put our finger on it, I think we all become frustrated with the idea of "pick and choose Judaism." We can't find a better word for it, but that's the idea we get. Let's discuss these philosophical foundations of the reform and conservative movements. (Of course, they have philosophies on other issues as well, but I think these are the main sticking points for many converts.)
A) The Reform Movement. I heard our local reform rabbi (a very educated and charismatic woman!) give a lecture about the current state of reform Judaism almost exactly 1 year ago. If I remember correctly, there had very recently been some important philosophical changes adopted by the movement, but I don't recall if they were related to this idea. She explained that the current reform approach to halacha is that 1) mitzvot between man and G-d is optional (each person should adopt the practices he or she finds meaningful) and that 2) mitzvot between man and man is mandatory.
B) The Conservative Movement. The conservative movement holds that halacha is mandatory, just as the orthodox do. However, they have a different interpretation of the individual mitzvot. How does the movement make rulings? There is a head group of rabbis of the conservative movement who come together and debate halachic issues, much like the Supreme Court of the United States does on secular cases. And like the Supreme Court, they write an official opinion handing down the majority ruling, but including separate minority opinions. At some point early in the conservative movement, it was decided that each congregation could choose which opinion to follow, whether it was the majority or a minority ruling. For instance, everyone knows that the conservative movement allows driving on Shabbat. Did you know that was only a minority opinion?? (Contrast: in orthodoxy, there is no group to decide. Each person must follow his or her own rabbi's rulings, as he follows the rulings of his own rabbis.) As an orthodox rabbi once told me (paraphrasing from memory), "Once you can follow the minority opinion, you can do whatever you want because eventually someone will write the opinion you want to follow."
What do these two philosophies come down to? There is no right answer. No, that wasn't a rhetorical question; that's the answer I'm giving. Curious converts with an overdose of logic become bothered by the lack of line-drawing by halacha without a ruling. While the orthodox may seem crazy from the outsider's perspective, you have to give them credit for something: they all agree that all the halacha is obligatory. Where they disagree is how that halacha should be followed. That's a much firmer logical ground to stand on, no matter what you think of halacha. I think liberal-turned-orthodox converts generally value consistency in philosophy. If the orthodox's greatest "logical fallacy" is that they think all the laws come straight from G-d and are mandatory, that's a much more consistent and tenable stance than that of the other two big movements. Orthodoxy follows its philosophy to its natural conclusion, whether or not one thinks that is a good place to end up :)
Why don't more liberal converts seek an orthodox conversion?
Quite frankly, many are probably happy where they are. More power to them! They're an inspiration to their communities, and a nourishment to their community's neshamas :)
As for the rest. This is probably an un-PC response. I suspect that it has a lot to do with the Jewish partners of converts. I think batei din (orthodox or liberal) are the only ones who realize the true importance of the significant other. Converts usually meet a Jewish significant other either before, during, or relatively soon after conversion, but the convert may not have completely grown into his or her "Jewish skin." Speaking from personal experience, wanting to grow in Judaism with an apathetic Jewish partner can bring out the foundational cracks in a relationship in ways normally only the birth of a first child can do. As they say, "an argument about the drapes is rarely about the drapes"! Then you're faced with a pretty awful decision: do you want to risk your relationship on religious changes you might not even like? It sure is a lot safer to stay in a religious place that no longer quite fits than to risk your relationship, particularly if you already have children! I do not envy that position.