Bi-sexual guys are hiding because of pressures from their societies especially from a conservative society like the United States.
I wouldn't say it's "conservatism" per se that causes most bi guys to stay (at least somewhat) closeted, although this plays a factor. The fact is, though, that there are some very open minded people out there that have no antiquated religious or moral qualms against same-sex love but hold unfounded stereotypes about bi men nonetheless:
1. That we're "really just gay and don't want to admit it yet" (yes, there are guys that are like this and it pisses me off to no end because they make life worse for us guys who truly are not gay)
2. That we're disease factories (yes, this is sometimes true but many of us aren't into unsafe sex)
3. That we cannot stay faithful (because we all know that straight men are notorious for never cheating...)
4. That we don't know what we want (just because I love coffee doesn't mean I don't like tea)
5. Bullshit ad infinitum (you get the picture...)
I mean, yes, some guys fit some or all of the stereotypes, but individually we vary as much as any other group.
If people had realistic views of bisexuality and didn't automatically group us into inaccurate boxes I would be a lot more open about my preferences. That's not to say I'm totally in the closet. My sister, cousin, pretty much all the close friends and girlfriends I've ever had and quite a few other people I've known have been privy to this info, and I'd answer truthfully to almost anyone who asked. Still, most of the time at work or in public I downplay my bi side quite a bit and I wish I didn't have to.
Oh yeah, that study Swede82 cited is absurdly easy to pick apart and one of the worst examples of "research" that I've ever seen. Either he's lying about having a Ph.D. or he has a vested interest in denying the existence of people that are not monosexual (just like the idiot who did the study). I mean, a fifth-grader who just learned about the scientific method could see through it. Sadly, the New York Times, one of the most influential newspapers in the country and (ostensibly) one of the more progressive ones, ran it completely uncritically.