Haram means illegal, I don't see any special and logical reason behind the bitcoin is illegal. Day by day it's gaining popularity among the people who engage in virtual work. So i 100% believe that its is totally contradict the people idea about haram of bitcoin because its 100% halal (legal).
If you are looking for shariah answer you better ask ulama and not here, you should also check the ulama religious background and his credibility regarding this matter.