Often, it's because buying a foreclosure can take a lot of additional time and effort.
But beyond that, you've already laid down a pretty questionable criterion: You only want to purchase a foreclosed home. Generally, buyers specify the number of bedrooms and baths. And/or location. And/or school district. And/or garage or swimming pool or fenced yard. Certainly price. All of those relate directly to the needs of the buyer.
But restricting your search to foreclosures? Why not tell a Realtor that you only want to look at houses where the house number ends in an even digit? Or houses on streets named for trees (Elm, Oak, Pine, etc.). Or houses previously owned by someone who graduated from college?
Well, that's what you're doing. You're introducing a requirement that has nothing to do with the quality of the home or how it matches your needs. You're even saying that your primary concern isn't value. After all, there are plenty of good values in non-foreclosures, and not all foreclosures are good values.
Now, having said all that, there should be plenty of Realtors out there who are willing to work with you. (I'm pretty sure some will respond to your question here.) But you're ruling a lot of them out--and you're trying to jump over an unnecessary hurdle--when you announce that you only want to purchase a foreclosure.
Hope that helps.