My theory is that this is - to a large degree - about the closet. Closeted bisexual men are far less likely to get tested, and if they do get an std, it is difficult for them to get treatment without exposing their hidden activities, which likely discourages a lot of men from taking action. If you are married (for example) and your wife is on the pill, you can't use a condom or she'll wonder why. If you try to talk to your doctor about std's, you don't know if he or she might say something to your spouse. It's difficult to even find out what the privacy laws of your state are in that regard, because simply asking about that makes people suspicious of you. If you are hiding, or even if you just don't trust straight society (or, in some cases, gay society) to respect you, then you don't have anywhere to talk honestly about std's, get tested, and get treatment without feeling exposed.
Please know I am not advocating for being in or staying in the closet, which is a whole different discussion. Without going into my entire story I'll just say that the closet is where I am for the foreseeable future, and I am not alone. I bring this up because whatever anyone's opinion of closeted bisexuals is, the fact is that we are here and in this situation.
I've managed to navigate this fairly well myself, and I'm super careful. But more than once I've found myself in very uncomfortable situations. I have never exposed anyone to anything, but I've careful as I was, I've caught a couple minor std's that I've had to deal with in secret. That is not easy. Doctors, testing, getting a prescription filled - all of these are not really as private as you might think. And if you are married, forget about trying to use your insurance ...
For several years in Los Angeles there was a clinic that catered to actors in the porn industry (male, female, all sexualities) and they also accepted clients who were not in that industry. This was perfect while it lasted: The staff was welcoming, friendly, nonjudgmental and privacy was extremely well accommodated. The doctors and nurses were very good, and their specialty was sexual health. Ironically, it was an AIDS activist group that got them shut down, as part of a bigger political crusade to pass a law requiring condom use in all porn shoots. Now those actors - and people like me - are on our own, and it is not simple. Just paying for a panel of tests becomes an issue, for example, because for the most discrete testing services you have to register online and pay with a credit card, then print out a form and take it to the blood draw center. This is a problem - potentially a deal breaker - for someone who doesn't want to get caught.
There are similar roadblocks to be navigated in almost every aspect of testing and treatment.
I'm sure that culture & community play a role in this as well, as 'patito' said above. If you are gay and out and have friends who are the same, HIV is still a big topic. It is not that way in the straight world, which is where closeted folks hide.