They do. But both are issues that effect men and so both are concerns for MRAs. The differences is that innocent men are being destroyed by women making false accusations, and most of those women (even when caught, even when they admit they lied) are not being sent to prison.
Rape is already a crime, and already taken seriously, already carries a steep penalty, etc. Most of male on male rape is done in prison where proof is hard to obtain (you are locked in a cell and controlled...you can't just go running to the hospital or police when it is over), and reporting is very low (because of retaliation...they are likely to be murdered). That is a hard one to help with. They are locked up in a cell with a rapist, or a murderer, etc. The laws are already there, but in practical reality, they are hard to apply in a prison setting. If you have any ideas of how we can reduce it, then by all means speak up.
False accusations on the other hand IS something MRAs can do something about. They can raise awareness to just how often this is happening, just how much it destroys men, just how bias the system is (assumed guilty until proven innocent instead of the other way around like required by law) and push for false accusers to be appropriately punished for a very serious crime...which is NOT being done now.
It is the difference between finding it difficult to apply the justice system to the prison system (male on male rape) and the justice system completely failing men (false accusations).
@Jess How can rape in prison be reduced? And why haven't feminists who claim they care about ALL rape done it? If feminists know what to do and they certainly have a lot more power than the fledging MRAs (with no money, no media support, etc), and feminists care about all rape, then why haven't they pushed for this mysterious solution??? It seems like you are the one just shooting your mouth off, because you would rather put down MRAs than actually reveal to the world your solution to prison rape.
ADD: Ok, let's put aside the fact the feminists, who claim to care about ALL rape, could have easily pressed for those things during their more than 4 decades as a political powerhouse. Do you have a plan for getting the money for more prisons, more walls, more guards, etc? Because politicians and penologists have been working on overcrowding for 50 years and haven't been able to figure that out. More taxes aren't popular and are very hard to get passed. And, how are you going to get data on prisoners who rape if the bulk of the rape victims don't come forward? How are you going to pay for segregation of them, or segregation and extra protection for the rape victims (and you will get a flood of both valid victims and a even bigger flood of those who lie just to get the segregation protection)? We can't house the prisoners we have now and are releasing them from prison earlier and earlier. How they heck are they supposed to find new solitary space for so many? You really don't have anything do you? If it was so simple why haven't feminists pressed for it (if only for women's prisons)? You got nothing but a bunch of empty platitudes. You think MRA can just cry like feminists to generate sympathy until someone figures out a way to help them. Well, if that is your plan, then you are going to have to convince your feminist heroes to not sit out a 5th decade, and get it done...because MRAs get no sympathy, no news coverage, etc when they cry.